[lldb] Prevent crash when adding a stop hook with --shlib

[JDevlieghere]

Summary

Currently, lldb crashes when adding a stop hook with --shlib because we unconditionally use the target in SymbolContextSpecifier::AddSpecification. Avoid the crash and add a little test.

rdar://68524781

[jingham]

This is fine, though the fact that it crashed when you tried to specify the shared library means we don't have any tests that the shared library gets copied over from the dummy target correctly. Might be nice to extend the test to have a "file not_called_test" and "break set -n main" and "run" to make sure the stop hook doesn't fire.

lldb/source/Symbol/SymbolContext.cpp

bool SymbolContextSpecifier::AddSpecification(const char *spec_string,
   switch (type) {
   case eNothingSpecified:
     Clear();
     break;
   case eModuleSpecified: {
     // See if we can find the Module, if so stick it in the SymbolContext.
     FileSpec module_file_spec(spec_string);
     ModuleSpec module_spec(module_file_spec);
-    lldb::ModuleSP module_sp(
-        m_target_sp->GetImages().FindFirstModule(module_spec));
+    lldb::ModuleSP module_sp = m_target_sp ?
+        m_target_sp->GetImages().FindFirstModule(module_spec) : nullptr;
     m_type |= eModuleSpecified;
     if (module_sp)
       m_module_sp = module_sp;
     else
       m_module_spec.assign(spec_string);
   } break;
   case eFileSpecified:
     // CompUnits can't necessarily be resolved here, since an inlined function

lldb/test/Shell/Commands/command-stop-hook-no-target.test

+# RUN: %lldb -b -o 'target stop-hook add --name test --shlib test' -o 'bt' | FileCheck %s
+# CHECK: Stop hook #1 added
+