minor: getbasetemp: dedent, improve assert (#5327)

minor: getbasetemp: dedent, improve assert

src/_pytest/tmpdir.py

@@ -60,7 +60,9 @@ class TempPathFactory(object):
 
     def getbasetemp(self):
         """ return base temporary directory. """
-        if self._basetemp is None:
+        if self._basetemp is not None:
+            return self._basetemp
+
         if self._given_basetemp is not None:
             basetemp = self._given_basetemp
             ensure_reset_dir(basetemp)
@@ -76,12 +78,10 @@ class TempPathFactory(object):
             basetemp = make_numbered_dir_with_cleanup(
                 prefix="pytest-", root=rootdir, keep=3, lock_timeout=LOCK_TIMEOUT
             )
-            assert basetemp is not None
+        assert basetemp is not None, basetemp
         self._basetemp = t = basetemp
         self._trace("new basetemp", t)
         return t
-        else:
-            return self._basetemp
 
 
 @attr.s