Avoid out-of-bounds imports read in wasm_instance_new

[aizu-m]

ASan, instantiating a 2-import module through the wasm C API with a 1-element imports vector:

==ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 8 ... in wasm_instance_new interp-wasm-c-api.cc:746
0 bytes after 8-byte region [...] (the caller's imports array)

Spotted while running modules through libwasm with a fixed imports list. wasm_instance_new copies the imports into a RefVec by looping i over the module's declared import count and reading imports->data[i]. Nothing checks the caller actually supplied that many, so a module whose import section declares more imports than the embedder passes reads past the end of the imports array and then derefs the wild pointer through ->I->self().

Instance::Instantiate already rejects a short list with a "not enough imports!" trap, but it runs after this loop, so the over-read happens first. Bounding the loop by imports->size lets the short case fall through to that existing trap instead. Valid and over-supplied lists build the same RefVec as before.

[sbc100]

IIRC that number of imports supplied should match the number expected by the module. Maybe we can instead error out here if they are not equal?

[aizu-m]

Makes sense. Changed it to an explicit equality check that traps with "wrong number of imports provided" and returns null before touching the array. One behavioural note: an over-supplied list used to be silently accepted, and now errors too. Reran the ASan repro (2-import module, 1 supplied): clean trap, no over-read. All the C API examples still pass.