Fix a SEGV when running --roctx-trace

There's a typo in RegisterApiCallback, roctx::cb_table.Get should be
roctx::cb_table.Set.

Change-Id: I47ec8ac666f783ff4e03f35d13e375e645899900

src/core/callback_table.h

@@ -45,6 +45,7 @@ template <uint32_t N> class CallbackTable {
 
   void Get(uint32_t id, activity_rtapi_callback_t* callback, void** arg) const {
     assert(id < N && "id is out of range");
+    assert(callback != nullptr && arg != nullptr && "invalid arguments");
     std::lock_guard lock(mutex_);
     std::tie(*callback, *arg) = callbacks_[id];
   }

src/roctx/roctx.cpp

@@ -230,7 +230,7 @@ PUBLIC_API bool RegisterApiCallback(uint32_t op, void* callback, void* arg) {
 
 PUBLIC_API bool RemoveApiCallback(uint32_t op) {
   if (op >= ROCTX_API_ID_NUMBER) return false;
-  roctx::cb_table.Get(op, NULL, NULL);
+  roctx::cb_table.Set(op, nullptr, nullptr);
   return true;
 }