Fix NULL dereference in the case of an unexpected extension from a server.

Due to a typo, when a server sent an unknown extension, the extension
number would be taken from a NULL structure rather than from the
variable of the same name that's in the local scope.

BUG=517935

Change-Id: I29d5eb3c56cded40f6155a81556199f12439ae06
Reviewed-on: https://boringssl-review.googlesource.com/5650
Reviewed-by: Adam Langley <agl@google.com>