gspawn: Don’t use g_ascii_isdigit() in async-signal-safe context

While `g_ascii_isdigit()` *is* currently async-signal-safe, it’s going
to be hard to remember to keep it that way if the implementation changes
in future.

It seems more robust to just reimplement it here, given that it’s not
much code.

See `man 7 signal-safety`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #2140

glib/gspawn.c

@@ -1185,8 +1185,7 @@ filename_to_fd (const char *p)
 
   while ((c = *p++) != '\0')
     {
-      /* FIXME: g_ascii_isdigit() is not necessarily async-signal-safe. */
-      if (!g_ascii_isdigit (c))
+      if (c < '0' || c > '9')
         return -1;
       c -= '0';