Avoid undefined behavior in detect_coding routines

Avoid undefined behavior in detect_coding routines

* src/coding.c (detect_coding): Always initialize all
components of detect_info, so that detect_coding_utf_8 etc.
do not have undefined behavior when they read detect_info.checked.
This bug is not likely to cause problems on real systems.
Problem found by GCC 12 -fanalyzer.
(detect_coding_system): Use consistent style with detect_coding
initialization.