fix: resolve undefined behavior (use-after-free) in Stack::Reserve

[yangzhg]

What changes were proposed in this pull request?

Refactored Stack::Reserve to calculate the current stack size before calling std::realloc.

Why are the changes needed?

This is a fix for Undefined Behavior.

The previous implementation invoked Size() (which reads buf_) after std::realloc(buf_, ...) had already executed. According to the C++ standard, the lifetime of the pointer passed to realloc ends immediately upon the call. Any subsequent access to buf_ (even just for pointer arithmetic in Size()) is invalid.

Although this issue was recently highlighted by GCC 12's stricter analysis (-Wuse-after-free), it represents a logical flaw that exists in all build environments.

Error context (exposed by GCC 12):

/include/sonic/internal/stack.h:45:56: warning: pointer used after ‘void* realloc(void*, size_t)’ [-Wuse-after-free]
   45 |   sonic_force_inline size_t Size() const { return top_ - buf_; }
      |                                                   ~~~~~^~~~~~