QDataBuffer: decrease size on shrink()

Having the size potentially be larger than the capacity can't
be considered anything other than a bug.

Pick-to: 6.5
Change-Id: Id059c0f2c7320f992d897011d7aa944c5cb86058
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/painting/qdatabuffer_p.h

@@ -89,13 +89,16 @@ public:
     }
 
     void shrink(qsizetype size) {
+        Q_ASSERT(capacity >= size);
         capacity = size;
         if (size) {
             buffer = (Type*) realloc(static_cast<void*>(buffer), capacity * sizeof(Type));
             Q_CHECK_PTR(buffer);
+            siz = std::min(siz, size);
         } else {
             free(buffer);
             buffer = nullptr;
+            siz = 0;
         }
     }