Skinny: do not try to create a wmem_strbuf whose size is bigger than the max size allowed

While we are at it, fix errors spotted by the pre-commit tools/SkinnyProtocolOptimized

Bug: 10409
Change-Id: Ic84632e0563f801239603534121e3487cf0d6d24
Reviewed-on: https://code.wireshark.org/review/3861
Reviewed-by: Evan Huus <eapache@gmail.com>