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/text-backend.c b/src/text-backend.c
index 7d2a064..e9578a4 100644
--- a/src/text-backend.c
+++ b/src/text-backend.c
@@ -938,11 +938,14 @@
 {
 	struct weston_config_section *section;
 	char *client;
+	int ret;
 
 	section = weston_config_get_section(text_backend->compositor->config,
 					    "input-method", NULL, NULL);
-	asprintf(&client, "%s/weston-keyboard",
-		 weston_config_get_libexec_dir());
+	ret = asprintf(&client, "%s/weston-keyboard",
+		       weston_config_get_libexec_dir());
+	if (ret < 0)
+		client = NULL;
 	weston_config_section_get_string(section, "path",
 					 &text_backend->input_method.path,
 					 client);