[Offload] Make AMDGPU plugin handle empty allocation properly

Summary:
`malloc(0)` and `free(nullptr)` are both defined by the standard but we
current trigger erros and assertions on them. Fix that so this works
with empty arguments.

Author: jhuber6

offload/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -420,7 +420,7 @@ struct AMDGPUMemoryManagerTy : public DeviceAllocatorTy {
     assert(PtrStorage && "Invalid pointer storage");
 
     *PtrStorage = MemoryManager->allocate(Size, nullptr);
-    if (*PtrStorage == nullptr)
+    if (Size && *PtrStorage == nullptr)
       return Plugin::error(ErrorCode::OUT_OF_RESOURCES,
                            "failure to allocate from AMDGPU memory manager");
 
@@ -429,8 +429,6 @@ struct AMDGPUMemoryManagerTy : public DeviceAllocatorTy {
 
   /// Release an allocation to be reused.
   Error deallocate(void *Ptr) {
-    assert(Ptr && "Invalid pointer");
-
     if (MemoryManager->free(Ptr))
       return Plugin::error(ErrorCode::UNKNOWN,
                            "failure to deallocate from AMDGPU memory manager");
@@ -1204,7 +1202,6 @@ struct AMDGPUStreamTy {
     ReleaseBufferArgsTy *Args = reinterpret_cast<ReleaseBufferArgsTy *>(Data);
     assert(Args && "Invalid arguments");
     assert(Args->MemoryManager && "Invalid memory manager");
-    assert(Args->Buffer && "Invalid buffer");
 
     // Release the allocation to the memory manager.
     return Args->MemoryManager->deallocate(Args->Buffer);

offload/test/api/omp_device_memory.c

@@ -2,6 +2,7 @@
 
 #include <omp.h>
 #include <stdio.h>
+#include <assert.h>
 
 int main() {
   const int N = 64;
@@ -24,4 +25,9 @@ int main() {
     printf("PASS\n");
 
   omp_free(device_ptr, llvm_omp_target_device_mem_alloc);
+
+  // Make sure this interface works.
+  void *ptr = omp_alloc(0, llvm_omp_target_device_mem_alloc);
+  assert(!ptr && "Ptr not (nullptr)");
+  omp_free(ptr, llvm_omp_target_device_mem_alloc);
 }