[Bug] possible SEGFAULT in reverse_utf8()

[albrechtd]

A (half-broken?) EMF file I fed into the lib caused a segfault in function reverse_utf8() due to the argument value of char *in being NULL, which in turn has been caused by calling

libemf2svg/src/lib/emf2svg_utils.c

Line 1399 in 821a40f

ret = fontindex_to_utf8((uint16_t *)in, size_in, (char **)&string,

which fills the local string with NULL.

Trivial fix: perform the call

libemf2svg/src/lib/emf2svg_utils.c

Line 1418 in 821a40f

reverse_utf8((char *)string, *size_out);

iff ((string != NULL) && (*size_out > 0U)).