[CritFix] Protect regexp matcher from regexps with empty patterns

Issue: #4885
Closes: #4885

src/libserver/re_cache.c

@@ -632,6 +632,11 @@ rspamd_re_cache_process_pcre(struct rspamd_re_runtime *rt,
 			if (max_hits > 0 && r >= max_hits) {
 				break;
 			}
+
+			if (start >= end) {
+				/* We found all matches, so no more hits are possible (protect from empty patterns) */
+				break;
+			}
 		}
 
 		rt->results[id] += r;

src/libutil/multipattern.c

@@ -717,6 +717,10 @@ int rspamd_multipattern_lookup(struct rspamd_multipattern *mp,
 										&end,
 										TRUE,
 										NULL)) {
+				if (start >= end) {
+					/* We found all matches, so no more hits are possible (protect from empty patterns) */
+					break;
+				}
 				if (rspamd_multipattern_acism_cb(i, end - in, &cbd)) {
 					goto out;
 				}

src/lua/lua_regexp.c

@@ -543,6 +543,11 @@ lua_regexp_search(lua_State *L)
 				}
 
 				matched = TRUE;
+
+				if (start >= end) {
+					/* We found all matches, so no more hits are possible (protect from empty patterns) */
+					break;
+				}
 			}
 
 			if (!matched) {
@@ -749,7 +754,7 @@ lua_regexp_split(lua_State *L)
 					lua_rawseti(L, -2, ++i);
 					matched = TRUE;
 				}
-				else if (start == end) {
+				else if (start >= end) {
 					break;
 				}
 				old_start = end;