AST-2018-006: Properly handle WebSocket frames with 0 length payload.

In ast_websocket_read() we were not adequately checking that the
payload_len was non-zero before passing it to ws_safe_read(). Calling
ws_safe_read with a len argument of 0 will result in a busy loop until
the underlying socket is closed.

ASTERISK-27658 #close

Change-Id: I9d59f83bc563f711df1a6197c57de473f6b0663a

res/res_http_websocket.c

@@ -488,13 +488,20 @@ const char * AST_OPTIONAL_API_NAME(ast_websocket_session_id)(struct ast_websocke
  * Note during the header parsing stage we try to read in small chunks just what we need, this
  * is buffered data anyways, no expensive syscall required most of the time ...
  */
-static inline int ws_safe_read(struct ast_websocket *session, char *buf, int len, enum ast_websocket_opcode *opcode)
+static inline int ws_safe_read(struct ast_websocket *session, char *buf, size_t len, enum ast_websocket_opcode *opcode)
 {
 	ssize_t rlen;
 	int xlen = len;
 	char *rbuf = buf;
 	int sanity = 10;
 
+	ast_assert(len > 0);
+
+	if (!len) {
+		errno = EINVAL;
+		return -1;
+	}
+
 	ao2_lock(session);
 	if (!session->stream) {
 		ao2_unlock(session);
@@ -608,9 +615,12 @@ int AST_OPTIONAL_API_NAME(ast_websocket_read)(struct ast_websocket *session, cha
 			return -1;
 		}
 
-		if (ws_safe_read(session, *payload, *payload_len, opcode)) {
-			return -1;
+		if (*payload_len) {
+			if (ws_safe_read(session, *payload, *payload_len, opcode)) {
+				return -1;
+			}
 		}
+
 		/* If a mask is present unmask the payload */
 		if (mask_present) {
 			unsigned int pos;