hmac: Fix memory leak
* src/hmac.c: Release HMAC256 context
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>