[Dexter] Continue sprinking no-location fixes

Example failure:

http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/3255/testReport/junit/debuginfo-tests/dexter-tests/asan_c/

The tests themselves seem to be working, it's just unexplored paths within
dexter that are flaking out.

Author: jmorse

debuginfo-tests/dexter/dex/command/commands/DexExpectWatchBase.py

@@ -168,7 +168,7 @@ def eval(self, step_collection):
         for step in step_collection.steps:
             loc = step.current_location
 
-            if (loc and os.path.exists(loc.path) and
+            if (loc.path and os.path.exists(loc.path) and
                 os.path.exists(self.path) and
                 os.path.samefile(loc.path, self.path) and
                 loc.lineno in self.line_range):