Segfault when processing some fonts

[wch]

The change in #13 caused this message to be printed for some fonts when calling extrafont::font_import(). Here is a font that causes the problem:

FiraSans-Bold.ttf.zip

This is an example of the error message:

> font_import('foo', prompt=F)
Scanning ttf files in foo ...
Extracting .afm files from .ttf files...
/Users/winston/Projects/Rttf2pt1/foo/FiraSans-Bold.ttfsh: line 1: 75065 Segmentation fault: 11  '/Users/winston/R/4.1/Rttf2pt1/exec//ttf2pt1' -a -GfAe '/Users/winston/Projects/Rttf2pt1/foo/FiraSans-Bold.ttf' '/var/folders/vd/0_g4hj6d7kq_fw5gd_r0ml5w0000gn/T//Rtmp4zq4o9/fonts/FiraSans-Bold' 2>&1
 : No FontName. Skipping.
Found FontName for 0 fonts.
Scanning afm files in /Users/winston/R/4.1/extrafontdb/metrics
Warning message:
In system2(enc2native(ttf2pt1), c(args, shQuote(ttfiles[i]), shQuote(tmpfiles[i])),  :
  running command ''/Users/winston/R/4.1/Rttf2pt1/exec//ttf2pt1' -a -GfAe '/Users/winston/Projects/Rttf2pt1/foo/FiraSans-Bold.ttf' '/var/folders/vd/0_g4hj6d7kq_fw5gd_r0ml5w0000gn/T//Rtmp4zq4o9/fonts/FiraSans-Bold' 2>&1' had status 139

Notably, there was a Segmentation fault: 11 message which was easy to miss. Running with lldb, I saw this:

$ lldb -- ./src/ttf2pt1/ttf2pt1 -a -G fAe foo/FiraSans-Bold.ttf 
(lldb) target create "./src/ttf2pt1/ttf2pt1"
Current executable set to '/Users/winston/Projects/Rttf2pt1/src/ttf2pt1/ttf2pt1' (arm64).
(lldb) settings set -- target.run-args  "-a" "-G" "fAe" "foo/FiraSans-Bold.ttf"
(lldb) r
Process 75975 launched: '/Users/winston/Projects/Rttf2pt1/src/ttf2pt1/ttf2pt1' (arm64)
Using language 'latin1' for Unicode fonts
Auto-detected front-end parser 'ttf'
 (use ttf2pt1 -p? to get the full list of available front-ends)
Processing file foo/FiraSans-Bold.ttf
Some font name strings are in Unicode, may not show properly
numglyphs = 2708
Found Unicode Encoding
Process 75975 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x4000000000000000)
    frame #0: 0x0000000100014f54 ttf2pt1`fdotsegdist2(seg=0x000000016fdfe168, dot=0x000000016fdfe188) at pt1.c:5036:7
   5033		/* end of macro */
   5034	
   5035	
-> 5036		dx = x2 - x1;
   5037		dy = y2 - y1;
   5038	
   5039		if(fabs(dx) < FEPS) {
Target 0: (ttf2pt1) stopped.

I think the problem is that in #13, I changed the type of double seg[2][2] to double** seg, to avoid a false positive compiler warning. However, these types are not the interchangeable. double seg[2][2] can be replaced with double* seg[2], but these are not equivalent to double** seg.

The good news (I think) is that while the (spurious) warning existed in gcc 11, it is not present in gcc 12, at least if I understand this comment correctly:
https://stackoverflow.com/questions/69891976/gcc-11-gives-wstringop-overflow-when-no-string-operation-is-used#comment123547865_69891976

So hopefully CRAN is testing on gcc 12 and I can submit an updated version with these changes reverted.