Add NULL check after struct_field_read_integer calls

[hobostay]

Summary

• Add NULL pointer checks in struct_field_get_unsigned_int and struct_field_get_signed_int macros in src/0xc/std/struct.h

Details

The macros struct_field_get_unsigned_int and struct_field_get_signed_int both call struct_field_read_integer(), which can return NULL when struct_field_get_untyped returns NULL (i.e., when the field is out of bounds of the provided memory buffer).

Both macros currently dereference the result without checking for NULL:

iv_0xc = struct_field_read_integer((SF), (P), (L), &ivmem_0xc);
// iv_0xc may be NULL here!
switch ((SF)->sf_size) {
case sizeof(uint8_t):
    v_0xc = iv_0xc->iv_int8.vu;  // NULL dereference

This adds a NULL check with a descriptive panic message to prevent undefined behavior from a NULL pointer dereference.

Test plan

• Existing test suite passes (make test)
• The change is defensive — it adds a check that triggers only when bounds-checking fails

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