fix asprintf warnings

Fix recently introduced compiler warnings:

desktop-shell/shell.c: In function 'shell_configuration':
desktop-shell/shell.c:588:10: warning: ignoring return value of
'asprintf', declared with attribute warn_unused_result [-Wunused-result]

src/screenshooter.c: In function ‘screenshooter_binding’:
src/screenshooter.c:291:10: warning: ignoring return value of
‘asprintf’, declared with attribute warn_unused_result [-Wunused-result]

src/text-backend.c: In function ‘text_backend_configuration’:
src/text-backend.c:944:10: warning: ignoring return value of ‘asprintf’,
declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/screenshooter.c b/src/screenshooter.c
index af2c754..cafbf10 100644
--- a/src/screenshooter.c
+++ b/src/screenshooter.c
@@ -287,9 +287,15 @@
 {
 	struct screenshooter *shooter = data;
 	char *screenshooter_exe;
+	int ret;
 
-	asprintf(&screenshooter_exe, "%s/%s", weston_config_get_libexec_dir(),
-					      "/weston-screenshooter");
+	ret = asprintf(&screenshooter_exe, "%s/%s",
+		       weston_config_get_libexec_dir(),
+		       "/weston-screenshooter");
+	if (ret < 0) {
+		weston_log("Could not construct screenshooter path.\n");
+		return;
+	}
 
 	if (!shooter->client)
 		shooter->client = weston_client_launch(shooter->ec,