[lldb] Fix crash when completing register names after program exit

Previously the following would crash:
(lldb) run
Process 2594053 launched: '/tmp/test.o' (aarch64)
Process 2594053 exited with status = 0 (0x00000000)
(lldb) register read <tab>

As the completer assumed that the execution context would always
have a register context. After a program has finished, it does not.

Split out the generic parts of the test from the x86 specific tests,
and added "register info" to both.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D154413

lldb/source/Commands/CommandCompletions.cpp

@@ -611,6 +611,9 @@ void CommandCompletions::Registers(CommandInterpreter &interpreter,
 
   RegisterContext *reg_ctx =
       interpreter.GetExecutionContext().GetRegisterContext();
+  if (!reg_ctx)
+    return;
+
   const size_t reg_num = reg_ctx->GetRegisterCount();
   for (size_t reg_idx = 0; reg_idx < reg_num; ++reg_idx) {
     const RegisterInfo *reg_info = reg_ctx->GetRegisterInfoAtIndex(reg_idx);

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

@@ -736,13 +736,25 @@ class CommandLineCompletionTestCase(TestBase):
         self.runCmd("type synthetic add -x Hoo -l test")
         self.complete_from_to("type synthetic delete ", ["Hoo"])
 
-    @skipIf(archs=no_match(["x86_64"]))
-    def test_register_read_and_write_on_x86(self):
-        """Test the completion of the commands register read and write on x86"""
-
+    def test_register_no_complete(self):
         # The tab completion for "register read/write"  won't work without a running process.
         self.complete_from_to("register read ", "register read ")
         self.complete_from_to("register write ", "register write ")
+        self.complete_from_to("register info ", "register info ")
+
+        self.build()
+        self.runCmd("target create {}".format(self.getBuildArtifact("a.out")))
+        self.runCmd("run")
+
+        # Once a program has finished you have an execution context but no register
+        # context so completion cannot work.
+        self.complete_from_to("register read ", "register read ")
+        self.complete_from_to("register write ", "register write ")
+        self.complete_from_to("register info ", "register info ")
+
+    @skipIf(archs=no_match(["x86_64"]))
+    def test_register_read_and_write_on_x86(self):
+        """Test the completion of the commands register read and write on x86"""
 
         self.build()
         self.main_source_spec = lldb.SBFileSpec("main.cpp")