ENG-48: Crash caused by importing negative offset

Some faulty MusicXML files include directions with an offset that
points to a tick < 0. This imports, but then crashes as soon as any
command is done on that direction. This commit adds a check upon import
that truncates any negative offset with a larger magnitude than the
associated tick, preventing this issue in MusicXML import.

importexport/musicxml/importmxmlpass2.cpp

@@ -2390,6 +2390,21 @@ static Fraction calcTicks(const QString& text, int divs, MxmlLogger* logger, con
       return dura;
       }
 
+//---------------------------------------------------------
+//   preventNegativeTick
+//---------------------------------------------------------
+/**
+  Prevent an offset that would result in a negative tick (set offset to -tick instead, resulting in a tick of 0)
+ */
+
+static void preventNegativeTick(const Fraction& tick, Fraction& offset, MxmlLogger* logger)
+      {
+      if (tick + offset < Fraction(0, 1)) {
+            logger->logError(QString("illegal offset %1 at tick %2").arg(offset.ticks()).arg(tick.ticks()));
+            offset = -tick;
+            }
+      }
+
 
 void MusicXMLDelayedDirectionElement::addElem()
       {
@@ -2432,8 +2447,10 @@ void MusicXMLParserDirection::direction(const QString& partId,
       while (_e.readNextStartElement()) {
             if (_e.name() == "direction-type")
                   directionType(starts, stops);
-            else if (_e.name() == "offset")
+            else if (_e.name() == "offset") {
                   _offset = calcTicks(_e.readElementText(), divisions, _logger, &_e);
+                  preventNegativeTick(tick, _offset, _logger);
+                  }
             else if (_e.name() == "sound")
                   sound();
             else if (_e.name() == "staff") {
@@ -5262,8 +5279,10 @@ void MusicXMLParserPass2::harmony(const QString& partId, Measure* measure, const
                   fd = frame(defaultY, relativeY, hasTotalY);
             else if (_e.name() == "level")
                   skipLogCurrElem();
-            else if (_e.name() == "offset")
+            else if (_e.name() == "offset") {
                   offset = calcTicks(_e.readElementText(), _divs, _logger, &_e);
+                  preventNegativeTick(sTime, offset, _logger);
+                  }
             else if (_e.name() == "staff") {
                   int nstaves = _pass1.getPart(partId)->nstaves();
                   QString strStaff = _e.readElementText();