[js/webgpu] fix Conv2DMatMul shader's out-of-bound read (#23085)

### Description
<!-- Describe your changes. -->

Fix a bug caused by potential out-of-bound reads of `W` in the
Conv2DMatMul shader.

### Motivation and Context

Fixes #22983

js/web/lib/wasm/jsep/webgpu/ops/3rd-party/conv2d_mm_webgpu.ts

@@ -143,7 +143,21 @@ const conv2dCommonSnippet = (
     }
     return ${typeSnippet(innerElementSizeX, dataType)}(0.0);`;
 
-  const sampleW = `${getWSnippet(innerElementSizeW)}`;
+  const sampleW = isChannelsLast
+    ? fitInner && fitBOuter
+      ? getWSnippet(innerElementSizeW)
+      : `
+    let col = colIn * ${innerElementSizeW};
+    if (row < uniforms.dim_inner && col < uniforms.dim_b_outer) {
+      ${getWSnippet(innerElementSizeW)}
+    }
+    return ${typeSnippet(innerElementSizeW, dataType)}(0.0);`
+    : `
+    let col = colIn * ${innerElementSizeW};
+    if (row < uniforms.dim_inner && col < uniforms.dim_a_outer) {
+      ${getWSnippet(innerElementSizeW)}
+    }
+    return ${typeSnippet(innerElementSizeW, dataType)}(0.0);`;
 
   const resType = typeSnippet(innerElementSize, dataType);
   const aType = isChannelsLast ? typeSnippet(innerElementSizeX, dataType) : typeSnippet(innerElementSizeW, dataType);