In case the PRF's set_key() or allocate_bytes() method failed, skeyseed
was not initialized and the chunk_clear() call later caused a crash.
This could have happened with OpenSSL in FIPS mode when MD5 was
negotiated (and test vectors were not checked, in which case the PRF
couldn't be instantiated as the test vectors would have failed).
MD5 is not included in the default proposal anymore since 5.6.1, so
with recent versions this could only happen with configs that are not
valid in FIPS mode anyway.
Fixes: CVE-2018-10811
6d8a57342b