clang compiler warnings: Fix non-literal-null-conversion warnings

Clang will flag errors when a char pointer is set to '\0', as opposed to a
value that the char pointer points to. This patch fixes this warning
in a variety of locations.

Review: https://reviewboard.asterisk.org/r/4551

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4551.patch submitted by dkdegroot (License 6600)
........

Merged revisions 434187 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434188 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -19738,13 +19738,13 @@ static void cleanup_stale_contexts(char *new, char *old)
 	char *oldcontext, *newcontext, *stalecontext, *stringp, newlist[AST_MAX_CONTEXT];
 
 	while ((oldcontext = strsep(&old, "&"))) {
-		stalecontext = '\0';
+		stalecontext = NULL;
 		ast_copy_string(newlist, new, sizeof(newlist));
 		stringp = newlist;
 		while ((newcontext = strsep(&stringp, "&"))) {
 			if (!strcmp(newcontext, oldcontext)) {
 				/* This is not the context you're looking for */
-				stalecontext = '\0';
+				stalecontext = NULL;
 				break;
 			} else if (strcmp(newcontext, oldcontext)) {
 				stalecontext = oldcontext;

channels/chan_skinny.c

@@ -2185,13 +2185,13 @@ static void cleanup_stale_contexts(char *new, char *old)
 	char *oldcontext, *newcontext, *stalecontext, *stringp, newlist[AST_MAX_CONTEXT];
 
 	while ((oldcontext = strsep(&old, "&"))) {
-		stalecontext = '\0';
+		stalecontext = NULL;
 		ast_copy_string(newlist, new, sizeof(newlist));
 		stringp = newlist;
 		while ((newcontext = strsep(&stringp, "&"))) {
 			if (strcmp(newcontext, oldcontext) == 0) {
 				/* This is not the context you're looking for */
-				stalecontext = '\0';
+				stalecontext = NULL;
 				break;
 			} else if (strcmp(newcontext, oldcontext)) {
 				stalecontext = oldcontext;

res/stasis_recording/stored.c

@@ -470,7 +470,7 @@ int stasis_app_stored_recording_copy(struct stasis_app_stored_recording *src_rec
 	/* Drop the extension if specified, core will do this for us */
 	format = strrchr(dst_file, '.');
 	if (format) {
-		format = '\0';
+		*format = '\0';
 	}
 
 	/* See if any intermediary directories need to be made */