There is a potential double free in EVP_DigestInit_ex. This is believed
to be reached only as a result of programmer error - but we should fix it
anyway.

(Imported from upstream's e78dc7e279)

Change-Id: I1da7be7db7afcbe9f30f168df000d64ed73d7edd
Reviewed-on: https://boringssl-review.googlesource.com/7541
Reviewed-by: David Benjamin <davidben@google.com>