build: Fix compiler warnings/errors.

The compiler complained about a couple of variables that weren't
initialized but were being used. Initializing them to NULL resolves the
warnings/errors.

ASTERISK-28362 #close

Change-Id: I6243afc5459b416edff6bbf571b0489f6b852e4b

channels/chan_sip.c

@@ -11637,7 +11637,7 @@ static int process_sdp_a_text(const char *a, struct sip_pvt *p, struct ast_rtp_c
 				ast_verbose("Discarded description format %s for ID %u\n", mimeSubtype, codec);
 		}
 	} else if (!strncmp(a, red_fmtp, strlen(red_fmtp))) {
-		char *rest;
+		char *rest = NULL;
 		/* count numbers of generations in fmtp */
 		red_cp = &red_fmtp[strlen(red_fmtp)];
 		strncpy(red_fmtp, a, 100);

pbx/pbx_dundi.c

@@ -3071,7 +3071,7 @@ static char *dundi_show_hints(struct ast_cli_entry *e, int cmd, struct ast_cli_a
 	ast_cli(a->fd, FORMAT2, "Prefix", "Context", "Expiration", "From");
 
 	for (db_entry = db_tree; db_entry; db_entry = db_entry->next) {
-		char *rest;
+		char *rest = NULL;
 
 		if (ast_get_time_t(db_entry->data, &ts, 0, &length)) {
 			continue;