eap_proxy: Check sm != NULL more consistently

While it does not look like that eapol_sm_get_key() would ever be called
with sm == NULL, the current implementation is inconsistent on whether
that is allowed or not. Check sm != NULL consistently to avoid warnings
from static analyzers.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/eapol_supp/eapol_supp_sm.c

@@ -1523,7 +1523,7 @@ int eapol_sm_get_key(struct eapol_sm *sm, u8 *key, size_t len)
 	size_t eap_len;
 
 #ifdef CONFIG_EAP_PROXY
-	if (sm->use_eap_proxy) {
+	if (sm && sm->use_eap_proxy) {
 		/* Get key from EAP proxy */
 		if (sm == NULL || !eap_proxy_key_available(sm->eap_proxy)) {
 			wpa_printf(MSG_DEBUG, "EAPOL: EAP key not available");