Fix assertion error when no header field was decompressed

bug: 10279
Change-Id: Ibf88b1622318692ccf3904aede5d964e6028bdc7
Reviewed-on: https://code.wireshark.org/review/3087
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
(cherry picked from commit 1b8c4bae489caa26cfd5d7b68a25394f286607e4)
Reviewed-on: https://code.wireshark.org/review/3088