[Dexter] Cope better with empty source locations

When running a program, Dexter single steps if it's in one of the source
files under test, or free-runs if it isn't. Handle the circumstance where
the current source file simply isn't known.

Author: jmorse

debuginfo-tests/dexter/dex/debugger/DebuggerBase.py

@@ -153,6 +153,8 @@ def start(self):
     def in_source_file(self, step_info):
         if not step_info.current_frame:
             return False
+        if not step_info.current_location.path:
+            return False
         if not os.path.exists(step_info.current_location.path):
             return False
         return any(os.path.samefile(step_info.current_location.path, f) \