QByteArray::insert: do not assert that pos must be non-negative

The insert() overloads have generally a very wide contract. The very
next line accepts negative positions, so remove the related assert.

Change-Id: I89b67615c59287825942047a28572bf896cf30e3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qbytearray.cpp

@@ -1911,8 +1911,6 @@ QByteArray& QByteArray::append(char ch)
 static inline QByteArray &qbytearray_insert(QByteArray *ba,
                                             int pos, const char *arr, int len)
 {
-    Q_ASSERT(pos >= 0);
-
     if (pos < 0 || len <= 0 || arr == nullptr)
         return *ba;