vis_pipe: correctly return non-zero exit status

according to POSIX wait(3p) the return status needs to be checked
and the macro WEXITSTATUS(stat_val) should be used to get the actual
return value on a normal exit. POSIX doesn't specify the value of
stat_val on abnormal exit and thus vis_pipe() should just return
-1 as it does for other errors

closes martanne#1130: vis:pipe returns wrong exit status (when non-zero)

Thanks @pippi1otta for the report and suggestion.

vis.c

@@ -1960,7 +1960,10 @@ int vis_pipe(Vis *vis, File *file, Filerange *range, const char *argv[],
 	vis->interrupted = false;
 	vis->ui->terminal_restore(vis->ui);
 
-	return status;
+	if (WIFEXITED(status))
+		return WEXITSTATUS(status);
+
+	return -1;
 }
 
 static ssize_t read_buffer(void *context, char *data, size_t len) {