[LLDB/Python] Fix segfault on Python scripted entrypoints

The code that gets the ScriptInterpreter was not considering the
case that it receives a Lua interpreter.

Differential Revision: https://reviews.llvm.org/D92249

lldb/test/Shell/ScriptInterpreter/Python/scripted_breakpoint_lua.test

@@ -0,0 +1,8 @@
+# REQUIRES: python
+# UNSUPPORTED: lldb-repro
+#
+# RUN: cat %s | %lldb --script-language lua 2>&1 | FileCheck %s
+b main
+breakpoint command add -s python -o 'print(frame); return False'
+run
+# CHECK: frame #0