[lldb] Prevent crash when completing ambiguous subcommands

Fix a crash when trying to complete an ambiguous subcommand. Take `set s
tar` for example: for the subcommand `s` there's ambiguity between set
and show. Pressing TAB after this input currently crashes LLDB. The
problem is that we're trying to complete `tar` but give up at `s`
because of the ambiguity. LLDB doesn't expect the completed string to be
shorter than the current string and crashes when trying to eliminate the
common prefix.

rdar://111848598

Differential revision: https://reviews.llvm.org/D154643

Author: JDevlieghere

lldb/source/Commands/CommandObjectMultiword.cpp

@@ -274,10 +274,10 @@ void CommandObjectMultiword::HandleCompletion(CompletionRequest &request) {
 
   StringList new_matches;
   CommandObject *sub_command_object = GetSubcommandObject(arg0, &new_matches);
-  if (sub_command_object == nullptr) {
-    request.AddCompletions(new_matches);
+
+  // The subcommand is ambiguous. The completion isn't meaningful.
+  if (!sub_command_object)
     return;
-  }
 
   // Remove the one match that we got from calling GetSubcommandObject.
   new_matches.DeleteStringAtIndex(0);

lldb/test/API/functionalities/completion/TestCompletion.py

@@ -880,3 +880,11 @@ def test_complete_breakpoint_with_names(self):
         self.complete_from_to("breakpoint set -N n", "breakpoint set -N n")
         self.assertTrue(bp1.AddNameWithErrorHandling("nn"))
         self.complete_from_to("breakpoint set -N ", "breakpoint set -N nn")
+
+    def test_ambiguous_command(self):
+        """Test completing an ambiguous commands"""
+        self.complete_from_to("settings s", ['set', 'show'])
+
+    def test_ambiguous_subcommand(self):
+        """Test completing a subcommand of an ambiguous command"""
+        self.complete_from_to("settings s ta", [])