Add missing NULL checks when calling malloc(M_NOWAIT) in

bhnd_nv_strdup/bhnd_nv_strndup.

If malloc(9) failed during initial bhnd(4) attach, while allocating the root
NVRAM path string ("/"), the returned NULL pointer would be passed as the
destination to memcpy().

Reported by:	Ilja Van Sprundel <ivansprundel@ioactive.com>

sys/dev/bhnd/nvram/bhnd_nvram_private.h

@@ -91,6 +91,9 @@ bhnd_nv_strdup(const char *str)
 
 	len = strlen(str);
 	dest = malloc(len + 1, M_BHND_NVRAM, M_NOWAIT);
+	if (dest == NULL)
+		return (NULL);
+
 	memcpy(dest, str, len);
 	dest[len] = '\0';
 
@@ -105,6 +108,9 @@ bhnd_nv_strndup(const char *str, size_t len)
 
 	len = strnlen(str, len);
 	dest = malloc(len + 1, M_BHND_NVRAM, M_NOWAIT);
+	if (dest == NULL)
+		return (NULL);
+
 	memcpy(dest, str, len);
 	dest[len] = '\0';