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/desktop-shell/shell.c b/desktop-shell/shell.c
index 6103309..234849a 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -575,6 +575,7 @@
 	struct weston_config_section *section;
 	int duration;
 	char *s, *client;
+	int ret;
 
 	section = weston_config_get_section(shell->compositor->config,
 					    "screensaver", NULL, NULL);
@@ -585,8 +586,10 @@
 
 	section = weston_config_get_section(shell->compositor->config,
 					    "shell", NULL, NULL);
-	asprintf(&client, "%s/%s", weston_config_get_libexec_dir(),
-				   WESTON_SHELL_CLIENT);
+	ret = asprintf(&client, "%s/%s", weston_config_get_libexec_dir(),
+		       WESTON_SHELL_CLIENT);
+	if (ret < 0)
+		client = NULL;
 	weston_config_section_get_string(section,
 					 "client", &s, client);
 	free(client);