rocr: uninitialized pointer read in InitScratchPool

Initialized 'scratch_base' as a nullptr to avoid
uninitialized read in hsaKmtAllocMemory()

Change-Id: I3b0e67f3fd3b591e1d21d691f0777b1d1a059b73
Signed-off-by: Apurv Mishra <apurv.mishra@amd.com>

runtime/hsa-runtime/core/runtime/amd_gpu_agent.cpp

@@ -555,7 +555,7 @@ void GpuAgent::InitScratchPool() {
   }
 #endif
 
-  void* scratch_base;
+  void* scratch_base = nullptr;
   HSAKMT_STATUS err =
       hsaKmtAllocMemory(node_id(), max_scratch_len, flags, &scratch_base);
   assert(err == HSAKMT_STATUS_SUCCESS && "hsaKmtAllocMemory(Scratch) failed");