LPP: avoid doing an out of bound access in case the PER decoding gives an invalid value

Bug: 10773
Change-Id: I4fcde84d7be1e42729b6ec0763064e9885951468
Reviewed-on: https://code.wireshark.org/review/5729
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
(cherry picked from commit 8e96830156bea314207b97315ccebd605317f142)
Reviewed-on: https://code.wireshark.org/review/5730