gh-87092: avoid gcc warning on uninitialized struct field in assemble…

[corona10]

….c (part2)

Python/assemble.c: In function ‘_PyAssemble_MakeCodeObject’:
Python/assemble.c:130:9: warning: ‘handler.h_startdepth’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     int depth = handler->h_startdepth - 1;
         ^~~~~
Python/assemble.c:147:34: note: ‘handler.h_startdepth’ was declared here
     _PyCompile_ExceptHandlerInfo handler;
                                  ^~~~~~~

• Issue: Split compiler into code-gen, optimizer and assembler. #87092

[corona10]

@iritkatriel It will be the last one.