autoboot: Add check for result of malloc_cache_aligned()

Return value of a function 'malloc_cache_aligned'
is dereferenced at autoboot.c:207 without checking for NULL,
but it is usually checked for this function.

Found by RASU JSC.

Signed-off-by: Maks Mishin <maks.mishinFZ@gmail.com>
diff --git a/common/autoboot.c b/common/autoboot.c
index 5d33199..6f0aeae 100644
--- a/common/autoboot.c
+++ b/common/autoboot.c
@@ -167,6 +167,9 @@
 		sha_env_str = AUTOBOOT_STOP_STR_SHA256;
 
 	presskey = malloc_cache_aligned(DELAY_STOP_STR_MAX_LENGTH);
+	if (!presskey)
+		return -ENOMEM;
+
 	c = strstr(sha_env_str, ":");
 	if (c && (c - sha_env_str < DELAY_STOP_STR_MAX_LENGTH)) {
 		/* preload presskey with salt */