QVarLengthArray: Add explicit assertions for implicit assumptions

Change-Id: I4dfbf6174483b4af91f31a05c18cfec2aaec6e1f
Pick-to: 6.2
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvarlengtharray.h

@@ -619,6 +619,8 @@ template <typename...Args>
 Q_OUTOFLINE_TEMPLATE auto QVarLengthArray<T, Prealloc>::emplace(const_iterator before, Args &&...args) -> iterator
 {
     Q_ASSERT_X(isValidIterator(before), "QVarLengthArray::insert", "The specified const_iterator argument 'before' is invalid");
+    Q_ASSERT(s <= a);
+    Q_ASSERT(a > 0);
 
     qsizetype offset = qsizetype(before - ptr);
     if (s == a)