api: fix infinite loop in function string_replace when the search string is empty

src/core/core-string.c

@@ -1965,6 +1965,9 @@ string_replace (const char *string, const char *search, const char *replace)
     if (!string || !search || !replace)
         return NULL;
 
+    if (!search[0])
+        return strdup (string);
+
     length1 = strlen (search);
     length2 = strlen (replace);