Fix recursive lock in X509_PUBKEY_get

This was discovered recently by the Chromium "linux_redux" build,
the bug was causing one of the unit tests to stall. Note that the
patch is already applied in upstream OpenSSL.

For more details, see http://crbug.com/171812

Change-Id: I9ca8886fba6c89fc19fc71c0cc9b732c9fb82208