scryptenc: free the expanded AES key when a write fails

[woahwhattheheck]

Disclosure, per this repository's AGENTS.md: I am an LLM (Claude), submitting on behalf of the account owner. I am available to discuss this change and to revise it in response to review feedback.

Fixes #429.

scryptenc_file() and scryptdec_file_copy() free the crypto_aesctr stream
when a write to the output file fails partway through, but not the
struct crypto_aes_key * it was built from:

		if (fwrite(buf, 1, readlen, outfile) < readlen) {
			crypto_aesctr_free(AES);
			rc = SCRYPT_EWRFILE;
			goto err1;
		}

err1 in scryptenc_file() only zeroes dk, and err0 in
scryptdec_file_copy() only returns, so key_enc_exp never reaches
crypto_aes_key_free(). The crypto_aesctr_init() failure path a few lines
above each of these already frees it; this just makes the write-error path
match. The _buf variants have no fwrite() and are unaffected.

The change

Two lines, one in each function. After it, every exit from the scope of a
crypto_aes_key_expand() in this file frees the result.

Test

tests/13-write-error.sh is new. The write-error path in these two loops has
no coverage today, which is why make test USE_VALGRIND=1 does not catch this.
The test builds an input larger than stdio's buffer — so the failure lands
inside the loop rather than at the final flush — and points the output at
/dev/full, checking exit status 1 and the Error writing file message for
both enc and dec. It also confirms that encrypting the same large input to
a normal file still succeeds.

/dev/full is not portable, so the test does that part only if
[ -c /dev/full ] and [ -w /dev/full ]; elsewhere it runs the plain
large-file encryption check and returns. Explicit parameters plus -f keep
the key derivation cheap so the test does not measure CPU speed on every run.

Note on the test number

Numbered 13 so it does not collide with tests/11-info.sh from #426 or
tests/12-same-file.sh from #428. The ordering works whichever of the three
lands first; the only overlap between them is the adjacent line each adds to
EXTRA_DIST in Makefile.am. Happy to renumber.