Partially revert "ed25519: Fix segfault on verify"

This partially reverts commit 87fbc59.

# Conflicts:
#	tests/ed25519_test.c

src/pk/ec25519/tweetnacl.c

@@ -473,6 +473,7 @@ int tweetnacl_crypto_sign_open(int *stat, u8 *m,u64 *mlen,const u8 *sm,u64 smlen
 
   *stat = 0;
   if (*mlen < smlen) return CRYPT_BUFFER_OVERFLOW;
+  *mlen = -1;
   if (smlen < 64) return CRYPT_INVALID_ARG;
 
   if (unpackneg(q,pk)) return CRYPT_ERROR;