app_voicemail: Add blank argument to externnotify if no context argument

At least one call to run_externnotify provides a NULL context parameter and
because the snprintf statement doesn't account for a NULL context parameter,
it simply writes '(null)' to the arguments string instead. This patch makes
it write two quotes back to back for that argument instead in the event of
a NULL context.

(closes issue ASTERISK-18207)
Reported by: Barry L. Kline
Patches:
	modified from patch-20130306 uploaded by Karsten Wemheuer (License 5930)
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Merged revisions 384325 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 384326 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@384327 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_voicemail.c

@@ -5944,7 +5944,10 @@ static void run_externnotify(char *context, char *extension, const char *flag)
 		if (inboxcount2(ext_context, &urgentvoicemails, &newvoicemails, &oldvoicemails)) {
 			ast_log(AST_LOG_ERROR, "Problem in calculating number of voicemail messages available for extension %s\n", extension);
 		} else {
-			snprintf(arguments, sizeof(arguments), "%s %s %s %d %d %d &", externnotify, context, extension, newvoicemails, oldvoicemails, urgentvoicemails);
+			snprintf(arguments, sizeof(arguments), "%s %s %s %d %d %d &",
+				externnotify, S_OR(context, "\"\""),
+				extension, newvoicemails,
+				oldvoicemails, urgentvoicemails);
 			ast_debug(1, "Executing %s\n", arguments);
 			ast_safe_system(arguments);
 		}