There are two reasons:
  - If the mruby call stack is extended, the `ci` variable may become invalid.
  - The C language does not specify the order in which the left-hand and right-hand sides of an assignment expression are evaluated.
    Therefore, if the mruby data stack is extended, `ci->stack` may become invalid.

Summary of Changes

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This pull request enhances the robustness of the mruby virtual machine by modifying how the result of mrb_funcall() is handled. It specifically mitigates risks associated with the C language's unspecified evaluation order and potential call stack extensions, which could lead to invalid call information or stack pointers. The change ensures that critical VM state variables remain consistent and valid throughout function calls.

Highlights

Preventing Call Information Invalidation: The pull request addresses a potential issue where the ci (call information) variable could become invalid if the mruby call stack is extended during a mrb_funcall() operation.
Addressing C Language Evaluation Order: It resolves a concern regarding the unspecified evaluation order of left-hand and right-hand sides in C assignment expressions, which could lead to ci->stack becoming invalid if the mruby data stack is extended by mrb_funcall().
Safe Assignment of mrb_funcall() Result: The change introduces an intermediate variable to store the result of mrb_funcall() before assigning it to regs[a], ensuring that ci is re-fetched after the function call and before the assignment, thus maintaining data integrity.

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