res_http_websocket: Avoid reading past end of string

We read beyond the end of the buffer when copying the string out of the
buffer when we used ast_copy_string() because the original string was
not null terminated. Instead switch to ast_strndup() which does not
exhibit the same behavior.

ASTERISK-28975 #close

Change-Id: Ib4a75cffeb1eb8cf01136ef30306bd623e531a2a

res/res_http_websocket.c

@@ -1473,11 +1473,10 @@ int AST_OPTIONAL_API_NAME(ast_websocket_read_string)
 		}
 	}
 
-	if (!(*buf = ast_malloc(payload_len + 1))) {
+	if (!(*buf = ast_strndup(payload, payload_len))) {
 		return -1;
 	}
 
-	ast_copy_string(*buf, payload, payload_len + 1);
 	return payload_len + 1;
 }