Cover case in which result in high-level overridden operations have m…

…ultiple lines and wer not be taken into account for minimum period calculations

backtrader/linebuffer.py

@@ -548,9 +548,8 @@ def dopreinit(cls, _obj, *args, **kwargs):
         # Do not produce anything until the operation lines produce something
         _minperiods = [x._minperiod for x in args if isinstance(x, LineSingle)]
 
-        if not _minperiods:
-            mlines = [x.lines[0] for x in args if isinstance(x, LineMultiple)]
-            _minperiods = [x._minperiod for x in mlines]
+        mlines = [x.lines[0] for x in args if isinstance(x, LineMultiple)]
+        _minperiods += [x._minperiod for x in mlines]
 
         _minperiod = max(_minperiods or [1])