hmac: Fix memory leak

* src/hmac.c: Release HMAC256 context

Cherry-picked the master commit of:
6994d874af865cc6ba95a8a6eb5a8fe048e88e07

LeakSanitizer marks the allocation of this context as leaked. Since the
hmac binary is used during the build with --enable-hmac-binary-check,
this fails the build with AddressSanitizer/LeakSanitizer.

• Signed-off-by: Clemens Lang <cllang@redhat.com>