Fix memory leaks on error in x_x509a.c.

See also upstream's c8491de393639dbc4508306b7dbedb3872b74293.

Change-Id: I017fb137d6d93b6abb82fdb03f02be8292963d0d
Reviewed-on: https://boringssl-review.googlesource.com/5767
Reviewed-by: Adam Langley <agl@google.com>