BUG/MEDIUM: http: fix possible parser crash when parsing erroneous "http-request redirect" rules

Baptiste Assmann reported a bug affecting the "http-request redirect"
parser. It may randomly crash when reporting an error message if the
syntax is not OK. It happens that this is caused by the output error
message pointer which was not initialized to NULL.

This bug is 1.5-specific (introduced in dev17), no backport is needed.
diff --git a/src/proto_http.c b/src/proto_http.c
index e825167..f037b1e 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -8491,7 +8491,7 @@
 		cur_arg += 2;
 	} else if (strcmp(args[0], "redirect") == 0) {
 		struct redirect_rule *redir;
-		char *errmsg;
+		char *errmsg = NULL;
 
 		if ((redir = http_parse_redirect_rule(file, linenum, proxy, (const char **)args + 1, &errmsg)) == NULL) {
 			Alert("parsing [%s:%d] : error detected in %s '%s' while parsing 'http-request %s' rule : %s.\n",