Backport upstream commit:
http://git.savannah.gnu.org/cgit/gawk.git/commit/?id=982aa5c8

Fix to regex_internal.c from sed list.
Resolves: #825693.
Index: gawk-3.1.5/regex_internal.c
===================================================================
--- gawk-3.1.5.orig/regex_internal.c
+++ gawk-3.1.5/regex_internal.c
@@ -503,7 +503,7 @@ re_string_skip_chars (pstr, new_raw_idx,
        rawbuf_idx < new_raw_idx;)
     {
       int remain_len;
-      remain_len = pstr->len - rawbuf_idx;
+      remain_len = pstr->raw_len - rawbuf_idx;
       prev_st = pstr->cur_state;
       mbclen = mbrtowc (&wc, (const char *) pstr->raw_mbs + rawbuf_idx,
 			remain_len, &pstr->cur_state);