selftests: firmware: Fix ignored return val of asprintf() warn

Fix the following ingonred return val of asprintf() warn during
build:

cc -Wall -O2 fw_namespace.c -o ../tools/testing/selftests/firmware/fw_namespace
fw_namespace.c: In function ‘main’:
fw_namespace.c:132:2: warning: ignoring return value of ‘asprintf’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
132 | asprintf(&fw_path, "/lib/firmware/%s", fw_name);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

tools/testing/selftests/firmware/fw_namespace.c

···
 		die("mounting tmpfs to /lib/firmware failed\n");
 
 	sys_path = argv[1];
-	asprintf(&fw_path, "/lib/firmware/%s", fw_name);
+	if (asprintf(&fw_path, "/lib/firmware/%s", fw_name) < 0)
+		die("error: failed to build full fw_path\n");
 
 	setup_fw(fw_path);