Pacify C compiler

This eliminates a warning about a missing return
statement. The code was correct though, because
hex2num() is guarded by is_hexdigit().

postsrsd.c

   if (c >= '0' && c <= '9') return c - '0';
   if (c >= 'a' && c <= 'f') return c - 'a' + 10;
   if (c >= 'A' && c <= 'F') return c - 'A' + 10;
+  return 0;
 }
 
 static char num2hex (char c)