Fix leak in X509V3_EXT_add on error.

If it fails to be added to the list, the input should be freed. Found (sort of)
by scan-build; because of all the indirections and DYNAMIC flags, it doesn't
actually realize the leak's been fixed.

Change-Id: Idca10964e1ffb2ace1cea7f88d94693205d70d5b
Reviewed-on: https://boringssl-review.googlesource.com/2208
Reviewed-by: Adam Langley <agl@google.com>