FIPS refusals returned as success: short-key HMAC forgery #90

On FIPS builds, refused operations (GNUTLS_FIPS140_OP_NOT_APPROVED and other positive values) are returned to gnutls as success.

• With an HMAC key under 14 bytes, gnutls_hmac returns 0 without computing a MAC. The output equals HMAC with an all-zero key: keys "Jefe", "Jeff" and "anything" give the same tag, so a forged tag verifies.
• The same flaw is in the HKDF, PBKDF2 and RSA keygen error paths, and four curve checks return 0.
• Separately, CMAC/GMAC setkey frees a context that gnutls frees again (use-after-free and double free on a failed key setup).

Fixes and test (RFC 4231 TC2 with a 4-byte key): #87 (closed in favour of this issue).