Fix garbage free on malloc failure in ec_wNAF_mul.

PR#3595

(Imported from upstream's e04d426bf98ebb22abf0f15b6f09d333a6e8b2ad.)

Change-Id: I01a9d9bef7e911b3fb1565f8a582f5d6cc7d5537
Reviewed-on: https://boringssl-review.googlesource.com/2290
Reviewed-by: Adam Langley <agl@google.com>