[lldbtestsuite] Don't crash on `None` input for is_exe().

Now the function returns `False`, and the caller can take the
appropriate action (in this case, for --executable, reporting
that the file doesn't exist).

llvm-svn: 323566

lldb/packages/Python/lldbsuite/test/dotest.py

@@ -51,6 +51,8 @@ from ..support import seven
 
 def is_exe(fpath):
     """Returns true if fpath is an executable."""
+    if fpath == None:
+      return False
     return os.path.isfile(fpath) and os.access(fpath, os.X_OK)