Patch false maybe-uninitialized warning

[hyperpastel]

Cross compiling c3c for Linux i686 for void-pkgs fails due to a -Werror=maybe-uninitialized warning generated in src/compiler/sema_expr.c

2025-04-18T08:05:33.9357799Z /usr/bin/cc -DCURL_FOUND=0 -DLLVM_AVAILABLE=1 -DTB_AVAILABLE=0 -I/builddir/c3c-0.7.0/src -I/builddir/c3c-0.7.0/build -I/builddir/c3c-0.7.0/wrapper/include -I/builddir/c3c-0.7.0/dependencies/miniz -I /usr/include/libxml2 -DNDEBUG -fstack-clash-protection -D_FORTIFY_SOURCE=2 -mtune=i686 -O2    -ffile-prefix-map=/builddir/c3c-0.7.0/build=. -std=gnu11   -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -pthread -Wall -Werror -Wno-unknown-pragmas -Wno-unused-result -Wno-unused-function -Wno-unused-variable -Wno-unused-parameter -MD -MT CMakeFiles/c3c.dir/src/compiler/sema_expr.c.o -MF CMakeFiles/c3c.dir/src/compiler/sema_expr.c.o.d -o CMakeFiles/c3c.dir/src/compiler/sema_expr.c.o -c /builddir/c3c-0.7.0/src/compiler/sema_expr.c
2025-04-18T08:05:33.9363205Z In function 'sema_expr_analyse_swizzle',
2025-04-18T08:05:33.9363970Z     inlined from 'sema_expr_analyse_access' at /builddir/c3c-0.7.0/src/compiler/sema_expr.c:5270:11:
2025-04-18T08:05:33.9365451Z /builddir/c3c-0.7.0/src/compiler/sema_expr.c:5009:28: error: 'index' may be used uninitialized [-Werror=maybe-uninitialized]
2025-04-18T08:05:33.9366360Z  5009 |                 if ((index ^ val) & 0x10)
2025-04-18T08:05:33.9366794Z       |                     ~~~~~~~^~~~~~
2025-04-18T08:05:33.9367437Z /builddir/c3c-0.7.0/src/compiler/sema_expr.c: In function 'sema_expr_analyse_access':
2025-04-18T08:05:33.9368361Z /builddir/c3c-0.7.0/src/compiler/sema_expr.c:4996:13: note: 'index' was declared here
2025-04-18T08:05:33.9369023Z  4996 |         int index;
2025-04-18T08:05:33.9369345Z       |             ^~~~~
2025-04-18T08:05:33.9369715Z cc1: all warnings being treated as errors

This PR addresses this by default initializing index to 0

[lerno]

Thank you.