Heap buffer overflow from the fix to issue 83

[m-lw]

The fix for #83 changed the code to insert 2 chars, but the call to adjbuf just above it only allows for 1 char. This can cause a heap buffer overflow.

E.g. in git master on CentOS 6.10:
$ echo \\ | env LANG=C MALLOC_CHECK_=1 ./a.out '/[[:cntrl:]01234567[:graph:]]/' *** glibc detected *** a.out: realloc(): invalid pointer: 0x00000000008c5be0 *** \ $

[plan9]

interesting, thank you for spotting.