[mod_redirect,mod_rewrite] fix segfault w/ invalid syntax (fixes #2892)

(thx nti)

x-ref:
  "Segmentation fault with invalid lighttpd.conf syntax"
  https://redmine.lighttpd.net/issues/2892

src/keyvalue.c

@@ -122,7 +122,7 @@ static void pcre_keyvalue_buffer_append_match(buffer *b, const char **list, int
 static void pcre_keyvalue_buffer_append_ctxmatch(buffer *b, pcre_keyvalue_ctx *ctx, unsigned int num, int flags) {
     const struct cond_cache_t * const cache = ctx->cache;
     if (!cache) return; /* no enclosing match context */
-    if (num < (unsigned int)cache->patterncount) {
+    if ((int)num < cache->patterncount) {
         const int off = cache->matches[(num <<= 1)]; /*(num *= 2)*/
         const int len = cache->matches[num+1] - off;
         burl_append(b, cache->comp_value->ptr + off, (size_t)len, flags);