chan_websocket: Fix buffer overrun when processing TEXT websocket frames.

ast_websocket_read() receives data into a fixed 64K buffer then continually
reallocates a final buffer that, after all continuation frames have been
received, is the exact length of the data received and returns that to the
caller.  process_text_message() in chan_websocket was attempting to set a
NULL terminator on the received payload assuming the payload buffer it
received was the large 64K buffer.  The assumption was incorrect so when it
tried to set a NULL terminator on the payload, it could, depending on the
state of the heap at the time, cause heap corruption.

process_text_message() now allocates its own payload_len + 1 sized buffer,
copies the payload received from ast_websocket_read() into it then NULL
terminates it prevent the possibility of the overrun and corruption.

Resolves: #1384

channels/chan_websocket.c

@@ -421,12 +421,14 @@ static int process_text_message(struct websocket_pvt *instance,
 	}
 
 	/*
-	 * This is safe because the payload buffer is always >= 8K
-	 * even with LOW_MEMORY defined and we've already made sure the
-	 * command is less than 128 bytes.
+	 * Unfortunately, payload is not NULL terminated even when it's
+	 * a TEXT frame so we need to allocate a new buffer, copy
+	 * the data into it, and NULL terminate it.
 	 */
-	payload[payload_len] = '\0';
-	command = ast_strip(ast_strdupa(payload));
+	command = ast_alloca(payload_len + 1);
+	memcpy(command, payload, payload_len); /* Safe */
+	command[payload_len] = '\0';
+	command = ast_strip(command);
 
 	ast_debug(4, "%s: WebSocket %s command received\n",
 		ast_channel_name(instance->channel), command);