libcrypto: Fix memory leak in evp_final()

Fixes T116

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit a280747462)

src/libcrypto.c

@@ -196,6 +196,7 @@ void evp_update(EVPCTX ctx, const void *data, unsigned long len)
 void evp_final(EVPCTX ctx, unsigned char *md, unsigned int *mdlen)
 {
     EVP_DigestFinal(ctx, md, mdlen);
+    EVP_MD_CTX_free(ctx);
 }
 #endif