output_cmd_background: fix no file + valid mode

If output_cmd_background is given a valid mode as the first argument,
then there is no file given and an error should be returned.

join_args should not be called with an argc of zero since it sets the
last character to the null terminator. With an argc of zero, the length
is zero causing a heap buffer overflow when setting the byte before the
start of argv to '\0'. This probably will not ever generate a segfault,
but may cause data corruption to whatever is directly before it in
memory. To make other such cases easier to detect, this also adds a
sway_assert in join_args when argc is zero.

common/stringop.c

@@ -258,6 +258,9 @@ int unescape_string(char *string) {
 }
 
 char *join_args(char **argv, int argc) {
+	if (!sway_assert(argc > 0, "argc should be positive")) {
+		return NULL;
+	}
 	int len = 0, i;
 	for (i = 0; i < argc; ++i) {
 		len += strlen(argv[i]) + 1;