I misunderstood the OpenSSL semantics here. When receiving an error data
pointer via ERR_get_error_line_data and friends, although the error is
cleared, OpenSSL retains ownership of the data pointer. It's kept in the
cleared error until another error overrides it, or the whole error queue
is cleared.

It's pretty odd to have live pointers in empty errors so this change
allows an error queue to retain one data pointer. Thus the pointer
returned from ERR_get_error_line_data is valid until the next call to
ERR_get_error_line_data, or until the queue is freed.

From reviewing uses of the API, this is sufficient for all of them.

Change-Id: I73cb8e9c792452ae3c1a934ac8bbe8b5353b65b2
Reviewed-on: https://boringssl-review.googlesource.com/1880
Reviewed-by: Adam Langley <agl@google.com>