rocr: Add nullptr check in IterateExecutables

When an entry is deleted from the array, it's set to nullptr
but not removed. Most other functions that
iterate over the array check if the entry is nullptr
but this loop in IterateExecutables did not.

Change-Id: I763b361eea59f6df201bb86ead0234e95f2cf79c


[ROCm/ROCR-Runtime commit: f3664fd124]

projects/rocr-runtime/runtime/hsa-runtime/loader/executable.cpp

@@ -280,9 +280,11 @@ hsa_status_t AmdHsaCodeLoader::IterateExecutables(
   assert(callback);
 
   for (auto &exec : executables) {
-    hsa_status_t status = callback(Executable::Handle(exec), data);
-    if (status != HSA_STATUS_SUCCESS) {
-      return status;
+    if(exec != nullptr){
+      hsa_status_t status = callback(Executable::Handle(exec), data);
+      if (status != HSA_STATUS_SUCCESS) {
+        return status;
+      }
     }
   }