Assert that engine isn't nullptr before dereferencing

The engine pointer is guaranteed to be initialized in line 1838, so we
can assert that the correct value is passed into the function to fix
static analyzer warning 1d9b8ce922ee0891fb0d477dc17fdb8d.

Change-Id: I773bbaa579afec0d7a79d4393ee66fd26ba9629b
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/text/qtextlayout.cpp

@@ -1679,6 +1679,7 @@ namespace {
 
         inline void calculateRightBearing(QFontEngine *engine, glyph_t glyph)
         {
+            Q_ASSERT(engine);
             qreal rb;
             engine->getGlyphBearings(glyph, nullptr, &rb);