Skip loadExtRam when the buffer is empty (#74)

An empty Uint8Array is still truthy, so the constructor's
`if (extRamBuffer)` guard called loadExtRam even when there was no
save data to restore. For carts that don't advertise external RAM
(MBC5 with EXT_RAM_SIZE_NONE, ROM-only, etc.),
`_ext_ram_file_data_new` returns 0 and the wasm side then reads
from a null file-data ptr inside `_emulator_read_ext_ram`.
Sometimes that lands on valid wasm linear memory, sometimes it's
out of bounds depending on heap state from prior alloc/free
cycles — surfacing as an intermittent
"RuntimeError: memory access out of bounds" in long-running embed
scenarios that recreate the Emulator multiple times.

Tighten the guard to also require `extRamBuffer.byteLength > 0`.

Author: kwigbo

docs/simple.js

@@ -177,7 +177,7 @@ class Emulator {
     this.fps = 60;
     this.fastForward = false;
 
-    if (extRamBuffer) {
+    if (extRamBuffer && extRamBuffer.byteLength > 0) {
       this.loadExtRam(extRamBuffer);
     }