ARI: allow json content type with zero length body

When a request was received with a Content-type of json,
the body was sent for json parsing - even if it was zero
length.  This resulted in ARI requests failing that were
valid, such as a channel DELETE with no parameters.  The
code has now been changed to skip json parsing with zero
content length.

(closes issue SWP-6748)
Reported by: Samuel Galarneau
Review: https://reviewboard.asterisk.org/r/3360/
........

Merged revisions 410858 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410863 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/http.c

@@ -880,12 +880,16 @@ struct ast_json *ast_http_get_json(
 	}
 
 	buf = ast_http_get_contents(&content_length, ser, headers);
-	if (buf == NULL)
-	{
+	if (buf == NULL) {
 		/* errno already set */
 		return NULL;
 	}
 
+	if (!content_length) {
+		/* it is not an error to have zero content */
+		return NULL;
+	}
+
 	body = ast_json_load_buf(buf, content_length, NULL);
 	if (body == NULL) {
 		/* Failed to parse JSON; treat as an I/O error */