Check for null drill_unit from gerbv.

This can happen if there are no holes to drill.
This fixes #397

drill.cpp

@@ -722,7 +722,7 @@ map<int, drillbit> ExcellonProcessor::parse_bits() {
        currentDrill = currentDrill->next) {
     drillbit curBit;
     curBit.diameter = currentDrill->drill_size;
-    curBit.unit = string(currentDrill->drill_unit);
+    curBit.unit = currentDrill->drill_unit == nullptr ? "" : string(currentDrill->drill_unit);
     curBit.drill_count = currentDrill->drill_count;
 
     bits.insert(pair<int, drillbit>(currentDrill->drill_num, curBit));

integration_tests.py

@@ -75,6 +75,7 @@
                   "multivibrator_xy_offset",
                   "multivibrator_xy_offset_zero_start",
                   "multi_outline",
+                  "null_drill",
                   "overlapping_edge_cuts",
                   "round_pcb_3",
                   "round_pcb_4",

testing/gerbv_example/null_drill/expected/front.ngc

@@ -0,0 +1,184 @@
+( pcb2gcode 2.1.0 )
+( Software-independent Gcode )
+
+G94 ( Inches per minute feed rate. )
+G20 ( Units == INCHES. )
+
+G90 ( Absolute coordinates. )
+G00 S800 ( RPM spindle speed. )
+G01 F11.81102 ( Feedrate. )
+
+
+G00 Z0.78740 (Retract to tool change height)
+T0
+M5      (Spindle stop.)
+G04 P1.00000 (Wait for spindle to stop)
+(MSG, Change tool bit to mill diameter 0.02362in)
+M6      (Tool change.)
+M0      (Temporary machine stop.)
+M3 ( Spindle on clockwise. )
+G04 P1.00000 (Wait for spindle to get up to speed)
+G04 P0 ( dwell for no time -- G64 should not smooth over this point )
+G00 Z0.03937 ( retract )
+
+G00 X0.81279 Y-0.60413 ( rapid move to begin. )
+G01 F5.90551
+( Mill infeed pass 1/1 )
+G01 Z-0.01024
+G04 P0 ( dwell for no time -- G64 should not smooth over this point )
+G01 F11.81102
+G01 X0.81279 Y-0.60413
+G01 X0.81136 Y-0.60834
+G01 X0.80792 Y-0.61443
+G01 X0.80647 Y-0.61642
+G01 X0.80174 Y-0.62158
+G01 X0.79612 Y-0.62574
+G01 X0.79398 Y-0.62698
+G01 X0.78756 Y-0.62977
+G01 X0.77951 Y-0.63148
+G01 X0.77128 Y-0.63148
+G01 X0.76323 Y-0.62977
+G01 X0.75571 Y-0.62642
+G01 X0.75202 Y-0.62382
+G01 X0.66713 Y-0.62382
+G01 X0.66713 Y-0.65531
+G01 X0.66690 Y-0.65762
+G01 X0.66623 Y-0.65983
+G01 X0.66514 Y-0.66188
+G01 X0.66367 Y-0.66367
+G01 X0.66188 Y-0.66514
+G01 X0.65983 Y-0.66623
+G01 X0.65762 Y-0.66690
+G01 X0.65531 Y-0.66713
+G01 X0.58839 Y-0.66713
+G01 X0.58608 Y-0.66690
+G01 X0.58386 Y-0.66623
+G01 X0.58182 Y-0.66514
+G01 X0.58003 Y-0.66367
+G01 X0.57856 Y-0.66188
+G01 X0.57747 Y-0.65983
+G01 X0.57680 Y-0.65762
+G01 X0.57657 Y-0.65531
+G01 X0.57657 Y-0.58839
+G01 X0.57675 Y-0.58633
+G01 X0.57729 Y-0.58435
+G01 X0.57816 Y-0.58248
+G01 X0.57934 Y-0.58079
+G01 X0.58079 Y-0.57934
+G01 X0.58248 Y-0.57816
+G01 X0.58435 Y-0.57729
+G01 X0.58633 Y-0.57675
+G01 X0.58839 Y-0.57657
+G01 X0.62419 Y-0.57657
+G01 X0.62588 Y-0.57389
+G01 X0.62911 Y-0.57005
+G01 X0.63388 Y-0.56613
+G01 X0.63828 Y-0.56372
+G01 X0.64041 Y-0.56283
+G01 X0.64411 Y-0.56171
+G01 X0.64638 Y-0.56126
+G01 X0.65138 Y-0.56083
+G01 X0.75202 Y-0.56083
+G01 X0.75467 Y-0.55890
+G01 X0.75681 Y-0.55767
+G01 X0.76207 Y-0.55532
+G01 X0.76442 Y-0.55456
+G01 X0.77128 Y-0.55317
+G01 X0.77827 Y-0.55310
+G01 X0.78073 Y-0.55336
+G01 X0.78756 Y-0.55488
+G01 X0.79508 Y-0.55823
+G01 X0.80174 Y-0.56307
+G01 X0.80647 Y-0.56822
+G01 X0.80792 Y-0.57022
+G01 X0.81136 Y-0.57631
+G01 X0.81279 Y-0.58052
+G01 X0.81279 Y-0.60413
+G01 X0.81423 Y-0.60834
+G01 X0.81767 Y-0.61443
+G01 X0.81870 Y-0.61590
+G01 X0.81870 Y-0.68676
+G01 X0.81510 Y-0.69035
+G01 X0.65417 Y-0.69035
+G01 X0.65214 Y-0.68820
+G01 X0.65118 Y-0.68743
+G01 X0.64449 Y-0.68264
+G01 X0.63699 Y-0.67923
+G01 X0.63465 Y-0.67847
+G01 X0.62780 Y-0.67702
+G01 X0.62535 Y-0.67676
+G01 X0.61712 Y-0.67682
+G01 X0.60905 Y-0.67847
+G01 X0.60671 Y-0.67923
+G01 X0.59921 Y-0.68264
+G01 X0.59156 Y-0.68820
+G01 X0.58522 Y-0.69524
+G01 X0.58105 Y-0.70233
+G01 X0.58005 Y-0.70459
+G01 X0.57788 Y-0.71124
+G01 X0.57737 Y-0.71366
+G01 X0.57657 Y-0.72185
+G01 X0.57737 Y-0.73004
+G01 X0.57788 Y-0.73246
+G01 X0.58049 Y-0.74026
+G01 X0.58455 Y-0.74743
+G01 X0.58600 Y-0.74942
+G01 X0.59156 Y-0.75550
+G01 X0.59921 Y-0.76106
+G01 X0.60786 Y-0.76491
+G01 X0.61590 Y-0.76668
+G01 X0.61835 Y-0.76694
+G01 X0.62658 Y-0.76688
+G01 X0.63584 Y-0.76491
+G01 X0.64449 Y-0.76106
+G01 X0.65118 Y-0.75627
+G01 X0.65214 Y-0.75550
+G01 X0.65417 Y-0.75335
+G01 X0.82815 Y-0.75335
+G01 X0.83315 Y-0.75291
+G01 X0.83542 Y-0.75246
+G01 X0.83911 Y-0.75134
+G01 X0.84125 Y-0.75045
+G01 X0.84565 Y-0.74804
+G01 X0.85042 Y-0.74412
+G01 X0.87325 Y-0.72122
+G01 X0.87569 Y-0.71823
+G01 X0.87698 Y-0.71631
+G01 X0.87880 Y-0.71290
+G01 X0.87969 Y-0.71076
+G01 X0.88080 Y-0.70707
+G01 X0.88126 Y-0.70480
+G01 X0.88169 Y-0.69980
+G01 X0.88169 Y-0.61590
+G01 X0.88272 Y-0.61443
+G01 X0.88616 Y-0.60834
+G01 X0.88871 Y-0.60051
+G01 X0.88957 Y-0.59232
+G01 X0.88871 Y-0.58414
+G01 X0.88616 Y-0.57631
+G01 X0.88272 Y-0.57022
+G01 X0.88127 Y-0.56822
+G01 X0.87654 Y-0.56307
+G01 X0.86988 Y-0.55823
+G01 X0.86352 Y-0.55532
+G01 X0.86117 Y-0.55456
+G01 X0.85553 Y-0.55336
+G01 X0.85308 Y-0.55310
+G01 X0.84608 Y-0.55317
+G01 X0.83922 Y-0.55456
+G01 X0.83688 Y-0.55532
+G01 X0.83161 Y-0.55767
+G01 X0.82947 Y-0.55890
+G01 X0.82385 Y-0.56307
+G01 X0.81834 Y-0.56918
+G01 X0.81423 Y-0.57631
+G01 X0.81279 Y-0.58052
+
+G04 P0 ( dwell for no time -- G64 should not smooth over this point )
+G00 Z0.787402 ( retract )
+
+M5 ( Spindle off. )
+G04 P1.000000
+M9 ( Coolant off. )
+M2 ( Program end. )
+