[lldb] Fix a crasher when using the public API. (#80508)

A user found a crash when they would do code like:
```
(lldb) script
>>> target = lldb.SBTarget()
>>> lldb.debugger.SetSelectedTarget(target)
```

We were not checking if the target was valid in and it caused a crash..

Author: clayborg

lldb/source/Target/TargetList.cpp

@@ -532,9 +532,13 @@ void TargetList::SetSelectedTarget(uint32_t index) {
 }
 
 void TargetList::SetSelectedTarget(const TargetSP &target_sp) {
-  std::lock_guard<std::recursive_mutex> guard(m_target_list_mutex);
-  auto it = llvm::find(m_target_list, target_sp);
-  SetSelectedTargetInternal(std::distance(m_target_list.begin(), it));
+  // Don't allow an invalid target shared pointer or a target that has been
+  // destroyed to become the selected target.
+  if (target_sp && target_sp->IsValid()) {
+    std::lock_guard<std::recursive_mutex> guard(m_target_list_mutex);
+    auto it = llvm::find(m_target_list, target_sp);
+    SetSelectedTargetInternal(std::distance(m_target_list.begin(), it));
+  }
 }
 
 lldb::TargetSP TargetList::GetSelectedTarget() {

lldb/test/API/python_api/target/TestTargetAPI.py

@@ -526,3 +526,9 @@ def test_is_loaded(self):
                 target.IsLoaded(module),
                 "Running the target should " "have loaded its modules.",
             )
+
+    @no_debug_info_test
+    def test_setting_selected_target_with_invalid_target(self):
+        """Make sure we don't crash when trying to select invalid target."""
+        target = lldb.SBTarget()
+        self.dbg.SetSelectedTarget(target)