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

[corona10]

I noticed the warn while I built CPython with GCC8 + PGO + LTO.

gcc -pthread -c -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall    -fno-semantic-interposition -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none -g -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden -fprofile-use -fprofile-correction -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE -o Python/bltinmodule.o Python/bltinmodule.c
Python/assemble.c: In function ‘_PyAssemble_MakeCodeObject’:
Python/assemble.c:131:8: warning: ‘handler.h_preserve_lasti’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     if (handler->h_preserve_lasti) {
        ^
Python/assemble.c:147:34: note: ‘handler.h_preserve_lasti’ was declared here
     _PyCompile_ExceptHandlerInfo handler;
                                  ^~~~~~~
Python/assemble.c:132:15: warning: ‘handler.h_startdepth’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         depth -= 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]

Tests / Hypothesis Tests on Ubuntu (pull_request)

Unrelated to this PR: #22863 (comment)

[iritkatriel]

Thanks