r3.in.v5d: Fix -Wliteral-range warning (#2748)

Addresses a -Wliteral-range compiler warning, with the message:
"floating-point comparison is always false; constant cannot
be represented exactly in type 'float' "

raster3d/r3.in.v5d/main.c

@@ -138,7 +138,7 @@ void convert(char *openFile, RASTER3D_Region *region, int convertNull,
                 for (y = 0; y < region->rows; y++) {
                     for (x = 0; x < region->cols; x++) {
                         value = data1[cnt++];
-                        if (convertNull && (value == MISSING))
+                        if (convertNull && IS_MISSING(value))
                             Rast3d_set_null_value(&value, 1, FCELL_TYPE);
                         Rast3d_put_float(map, x, y, z, value);
                     }