Fix two unchecked mallocs in crypto/asn1.

Imported from upstream's 918bb86529. (The
remainder is in code we've since deleted.)

Change-Id: Ie878272114086ba60a0fd5eae059b641b00ec0c4
Reviewed-on: https://boringssl-review.googlesource.com/3831
Reviewed-by: Adam Langley <agl@google.com>

crypto/asn1/bio_ndef.c

@@ -170,6 +170,9 @@ static int ndef_prefix(BIO *b, unsigned char **pbuf, int *plen, void *parg)
 
 	derlen = ASN1_item_ndef_i2d(ndef_aux->val, NULL, ndef_aux->it);
 	p = OPENSSL_malloc(derlen);
+	if (p == NULL)
+		return 0;
+
 	ndef_aux->derbuf = p;
 	*pbuf = p;
 	derlen = ASN1_item_ndef_i2d(ndef_aux->val, &p, ndef_aux->it);
@@ -235,6 +238,9 @@ static int ndef_suffix(BIO *b, unsigned char **pbuf, int *plen, void *parg)
 
 	derlen = ASN1_item_ndef_i2d(ndef_aux->val, NULL, ndef_aux->it);
 	p = OPENSSL_malloc(derlen);
+	if (p == NULL)
+		return 0;
+
 	ndef_aux->derbuf = p;
 	*pbuf = p;
 	derlen = ASN1_item_ndef_i2d(ndef_aux->val, &p, ndef_aux->it);