Account for the MTU BIO_ctrls returning negative or overly large numbers.

BIO_ctrls do not have terribly well-defined return values on error. (Though the
existing ones seem to all return 0, not -1, on nonexistant operation.)

Change-Id: I08497f023ce3257c253aa71517a98b2fe73c3f74
Reviewed-on: https://boringssl-review.googlesource.com/2829
Reviewed-by: Adam Langley <agl@google.com>