Defensive code fix to prevent out of bounds memory copy in GetElementPtr

Author: Zorro666

renderdoc/driver/shaders/dxil/dxil_debug.cpp

@@ -4258,12 +4258,17 @@ bool ThreadState::ExecuteInstruction(DebugAPIWrapper *apiWrapper,
 
       result.type = baseType;
       result.rows = (uint8_t)countElems;
-      backingMemory += offset;
-      m_Memory.m_AllocPointers[resultId] = {ptrId, backingMemory, size};
 
       RDCASSERT(offset + size <= alloc.size);
-      RDCASSERT(size < sizeof(result.value.f32v));
-      memcpy(&result.value.f32v[0], backingMemory, size);
+      if(offset + size <= alloc.size)
+      {
+        backingMemory += offset;
+        m_Memory.m_AllocPointers[resultId] = {ptrId, backingMemory, size};
+
+        RDCASSERT(size < sizeof(result.value.f32v));
+        if(size < sizeof(ShaderValue))
+          memcpy(&result.value.f32v[0], backingMemory, size);
+      }
       break;
     }
     case Operation::Bitcast: