gcm_mode_encrypt_contiguous_blocks() can write out of bounds
Port of ZoL #9660:
If gcm_mode_encrypt_contiguous_blocks() is called more than once in succession, with the accumulated lengths being less than blocksize, ctx->copy_to will be incorrectly advanced. Later, if out is NULL, bcopy() will overflow ctx->gcm_copy_to since ctx->gcm_remainder_len is larger than the ctx->gcm_copy_to buffer can hold.
For ZoL the issue may be academic, since in all my testing I wasn't able to hit neither of both conditions needed to trigger it, but other consumers can easily do so.
The fix is to set ctx->copy_to only if it's not already set.