QTriangulator: make a primeForCount() precondition explicit

Coverity complains that the binary search algorithm can yield high =
32, which, when passed to primeForNumBits(), will overflow
prime_deltas[]. This is true, for negative 'count', or, more general,
if the MSB of 'count' is set.

Add a Q_ASSERT(count >= 0) to inform Coverity (and other readers of
the code) that this function is only expecting non-negative arguments.

Amends 4adf5e1a9ef4fe78f4b70b7462943246903f4f11, which didn't fully
solve the issue, at least not as far as Coverity is concerned.

Pick-to: 6.8 6.5
Coverity-Id: 11295
Task-number: QTBUG-134543
Change-Id: I994dd1d1850c6644188a9fc0b83973614dd51e6b
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit 536cd1ce20a59e4daaf3a09c115f2dc5ad9cbf91)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/painting/qtriangulator.cpp

@@ -418,6 +418,7 @@ static inline int primeForNumBits(int numBits)
 
 static inline int primeForCount(int count)
 {
+    Q_ASSERT(count >= 0); // Q_PRE
     int low = 0;
     int high = 32;
     for (int i = 0; i < 5; ++i) {