gcc 14 emits new warnings

[serhiy-storchaka]

New version of gcc is more picky and produces more warnings about potential bugs. Most of them are false alarms, but we should either silence warnings or rewrite the code in less ambiguous way.

Linked PRs

• gh-138264: Fix gcc 14 compiler warnings #138265
• [3.14] gh-138264: Fix gcc 14 compiler warnings (GH-138265) #138426

[serhiy-storchaka]

#138265 fixes all warnings except -Wchar-subscripts for which we should decide whether to silence them globally or change the type of variables and attributes used in indexing from int8_t to uint8_t or something other.