Added comment for the frag->reassembly == NULL case as per feedback from Emilia

Reviewed-by: Emilia Käsper <emilia@openssl.org>

(Imported from upstream's ea7cb5397457c59554155935b677a1dab23bd864)

Change-Id: Idd5ed233028c42d2b921deb424381aad88a0aa84
Reviewed-on: https://boringssl-review.googlesource.com/1433
Reviewed-by: Adam Langley <agl@google.com>