Prevent integer underflow if size is below 6

When processing 3GPP metadata, a subtraction operation may underflow and
lead to a rather large linear byteswap operation in the subsequent
framedata decoding code. Bound the 'size' value to prevent this from
occurring.

Bug: 20923261
Change-Id: I35dfbc8878c6b65cfe8b8adb7351a77ad4d604e5