Fix some Coverity warnings and false alarms

[vstinner]

I got access to a Coverity scan of Python 3.12.2 and there are about 67 warnings. Most of them seem to be false alarms, my team is still investigating the warnings.

I propose to make minor changes, when it makes sense, to make some false alarms quiet.

And propose fixes for real issues :-)

Linked PRs

• gh-120155: Initialize variables in _tkinter.c #120156
• gh-120155: Fix copy/paste error in HAVE_SUBOFFSETS_IN_LAST_DIM() #120228
• [3.12] gh-120155: Fix optimize_and_assemble_code_unit() error handling #120231
• gh-120155: Fix Coverity issue in zoneinfo load_data() #120232
• [3.13] gh-120155: Fix copy/paste error in HAVE_SUBOFFSETS_IN_LAST_DIM() (GH-120228) #120238
• [3.12] gh-120155: Fix copy/paste error in HAVE_SUBOFFSETS_IN_LAST_DIM() (GH-120228) #120240
• [3.13] gh-120155: Fix Coverity issue in zoneinfo load_data() (GH-120232) #120310
• [3.12] gh-120155: Fix Coverity issue in zoneinfo load_data() (GH-120232) #120311
• gh-120155: Add assertion to sre.c match_getindex() #120402
• [3.13] gh-120155: Add assertion to sre.c match_getindex() (GH-120402) #120409
• [3.12] gh-120155: Add assertion to sre.c match_getindex() (GH-120402) #120410
• gh-120155: Fix Coverity issue in parse_string() #120997
• [3.13] gh-120155: Fix Coverity issue in parse_string() (GH-120997) #121005
• [3.12] gh-120155: Fix Coverity issue in parse_string() (GH-120997) #121006