Fix crash with NUL character in path given to --file-list

[mcc0612mcc0612]

Description
If a path in the --file-list file contains a NUL character, cppcheck keeps opening the same folder over and over until it crashes

Crash Replication

mkdir -p d/s; printf 'd\0\n' > list.txt  && /path/to/cppcheck --file-list=list.txt

seg fault is expected

Proposed Patch
If a path in --file-list file contains NUL character, reject it.