pcre2posix: fix crash on recent regerror code

Since 0710ce2 (pcre2posix: avoid snprintf quirks in regerror (#333),
2023-11-15), a call for snprintf was replaced by a pair of strncpy
and buf[errbuf_size - 1] = 0, but it didn't account for the case
where errbuf_size == 0.

Make the code conditional to mimic the original logic and avoid
crashing.

src/pcre2posix.c

@@ -199,8 +199,11 @@ if (preg != NULL && (int)preg->re_erroffset != -1)
 else
   {
   len = strlen(message);
-  strncpy(errbuf, message, errbuf_size);
-  if (errbuf_size <= len) errbuf[errbuf_size - 1] = '\0';
+  if (errbuf_size != 0)
+    {
+    strncpy(errbuf, message, errbuf_size);
+    if (errbuf_size <= len) errbuf[errbuf_size - 1] = '\0';
+    }
   ret = (int)len;
   }