Fix bad access to detached ArrayBuffer in $growMemory. (#24524)

Consider:

```js
var m = new WebAssembly.Memory({initial: 64});
var b = m.buffer;
m.grow(64);
console.log(b.byteLength);
```
This will print `0` instead of the old memory size before growing (or
the new memory size after grow), because `b` got detached by the grow.

This caused bad assertions error print, and incorrect profiling.

To fix, avoid referencing a detached buffer.

Author: juj

src/lib/libcore.js

@@ -174,27 +174,24 @@ addToLibrary({
   // Grows the wasm memory to the given byte size, and updates the JS views to
   // it. Returns 1 on success, 0 on error.
   $growMemory: (size) => {
-    var b = wasmMemory.buffer;
-    var pages = ((size - b.byteLength + {{{ WASM_PAGE_SIZE - 1 }}}) / {{{ WASM_PAGE_SIZE }}}) | 0;
+    var oldHeapSize = wasmMemory.buffer.byteLength;
+    var pages = ((size - oldHeapSize + {{{ WASM_PAGE_SIZE - 1 }}}) / {{{ WASM_PAGE_SIZE }}}) | 0;
 #if RUNTIME_DEBUG
-    dbg(`growMemory: ${size} (+${size - b.byteLength} bytes / ${pages} pages)`);
-#endif
-#if MEMORYPROFILER
-    var oldHeapSize = b.byteLength;
+    dbg(`growMemory: ${size} (+${size - oldHeapSize} bytes / ${pages} pages)`);
 #endif
     try {
       // round size grow request up to wasm page size (fixed 64KB per spec)
       wasmMemory.grow({{{ toIndexType('pages') }}}); // .grow() takes a delta compared to the previous size
       updateMemoryViews();
 #if MEMORYPROFILER
       if (typeof emscriptenMemoryProfiler != 'undefined') {
-        emscriptenMemoryProfiler.onMemoryResize(oldHeapSize, b.byteLength);
+        emscriptenMemoryProfiler.onMemoryResize(oldHeapSize, wasmMemory.buffer.byteLength);
       }
 #endif
       return 1 /*success*/;
     } catch(e) {
 #if ASSERTIONS
-      err(`growMemory: Attempted to grow heap from ${b.byteLength} bytes to ${size} bytes, but got error: ${e}`);
+      err(`growMemory: Attempted to grow heap from ${oldHeapSize} bytes to ${size} bytes, but got error: ${e}`);
 #endif
     }
     // implicit 0 return to save code size (caller will cast "undefined" into 0