Add Q_ASSERT in qAllocMore() function.

The negative range test is no longer needed in QByteArray's autotest.

Task-number: QTBUG-33038
Change-Id: I4d77e78e485b3b64a31dffd99b9fe2c14420d0a2
(partial cherry picked from qtbase commit fb8be9905d5f3216edc3fbb72b8ce1c380737eac)
(partial cherry picked from qtbase commit 0ed3cf2a1c487387bbc958317c791c9c53cf5a16)
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>