improve logic of heap_type validation when ref.null

[Septa2112]

PR #4300 introduced the rationale for validating heap_type.

This patch moves the validation before the computation of type1 to prevent potential overflow.

[Septa2112]

Further improved the heap type check logic:

1. Moved the heap type validation ahead of the if (!is_byte_a_type...) check. Depending on whether heap_type >= 0, different methods are used to get type1.

2. The type_index check is now performed only when heap_type >= 0, to ensure that type1 is correctly retrieved in this case.

3. Why not checking type_index when heap_type < 0? Because in the current WAMR implementation, if wasm_is_valid_heap_type(heap_type) is true, then the condition if (!is_byte_a_type(type1) || wasm_is_type_multi_byte_type(type1)) will always be false.

[lum1n0us]

Guess heap_type should be type1.

[Septa2112]

Likewise, considering the same code context in function wasm_loader_prepare_bytecode when WASM_OP_REF_NULL , it might be better to keep the check for int32 heap_type here.

[lum1n0us]

If following the spec, s33 should be a type index. might rename it as type_index better.

[Septa2112]

I think it might not be appropriate to rename heap_type to type_idx here:

1. type_idx has already been defined as a uint32 at the beginning of this function.
2. Based on the surrounding lines of code in function wasm_loader_prepare_bytecode when WASM_OP_REF_NULL, I think it's clearer and more consistent to keep the name heap_type.
   

   wasm-micro-runtime/core/iwasm/interpreter/wasm_loader.c

   Lines 13154 to 13169 in 18d4227

   	pb_read_leb_int32(p, p_end, heap_type);
   	if (heap_type >= 0) {
   	if (!check_type_index(module, module->type_count, heap_type,
   	error_buf, error_buf_size)) {
   	goto fail;
   	}
   	wasm_set_refheaptype_typeidx(&wasm_ref_type.ref_ht_typeidx,
   	true, heap_type);
   	ref_type = wasm_ref_type.ref_type;
   	}
   	else {
   	if (!wasm_is_valid_heap_type(heap_type)) {
   	set_error_buf(error_buf, error_buf_size,
   	"unknown type");
   	goto fail;
   	}

[lum1n0us]

like above, in L839, rename type1 to heap_type or abs_heaptype.

[lum1n0us]

after wasm_is_valid_heap_type(), not sure we still need is_byte_a_type() and wasm_is_type_multi_byte_type().

[lum1n0us]

Another thing, L872 will read it again. Seems we can remove either one.