Only initialize indirect buffer to size

On iOS, we get a GPU hang/reset based upon the execution of the ClearDispatchIndirect kernel. The buffer is created using the 'NUM_FEATURE_VARIANTS' constant (currently 27), but we're dispatching a threadgroup of 64 threads, so we are presumably going out of bounds (Metal GPU errors are not terribly descriptive, but this is a common cause).

com.unity.render-pipelines.high-definition/HDRP/Lighting/LightLoop/cleardispatchindirect.compute

 
 RWBuffer<uint> g_DispatchIndirectBuffer : register( u0 );   // Indirect arguments have to be in a _buffer_, not a structured buffer
 
+#include "LightLoop.cs.hlsl"
+
+    // On iOS, we get a GPU hang/reset based upon the execution of the ClearDispatchIndirect kernel.
+    // The buffer is created using the 'NUM_FEATURE_VARIANTS' constant but we're dispatching a threadgroup of 64 threads, so we are presumably going out of bounds (Metal GPU errors are not terribly descriptive, but this is a common cause).
+    // In DirectX out-of-bounds reads are always zero and out-of-bounds writes are are a no op. 
+    if (dispatchThreadId >= NUM_FEATURE_VARIANTS)
+        return;
+
     g_DispatchIndirectBuffer[dispatchThreadId * 3 + 0] = 0; // ThreadGroupCountX
     g_DispatchIndirectBuffer[dispatchThreadId * 3 + 1] = 1; // ThreadGroupCountY
     g_DispatchIndirectBuffer[dispatchThreadId * 3 + 2] = 1; // ThreadGroupCountZ