QByteArray: don't recurse when needle or replacement alias *this

While the idea to delay the construction of the QVLAs until we know we
need them is elegant, we're doing a lot of checks twice or thrice this
way, so merely re-set the a, b pointers and otherwise continue
executing.

This requires us to default-construct the QVLAs before potentially
assign()ing to them, but that ought to be cheap, given the compiler
can see everything that's going on in QVLA.

Pick-to: 6.9 6.8 6.5
Change-Id: I4e42d5ab48711af91cc1785171524be5fb45ae2f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

src/corelib/text/qbytearray.cpp

@@ -2537,13 +2537,14 @@ QByteArray &QByteArray::replace(QByteArrayView before, QByteArrayView after)
         return *this;
 
     // protect against before or after being part of this
+    QVarLengthArray<char> pinnedNeedle, pinnedReplacement;
     if (QtPrivate::q_points_into_range(a, d)) {
-        QVarLengthArray copy(a, a + asize);
-        return replace(before, QByteArrayView{copy});
+        pinnedReplacement.assign(a, a + asize);
+        a = pinnedReplacement.data();
     }
     if (QtPrivate::q_points_into_range(b, d)) {
-        QVarLengthArray copy(b, b + bsize);
-        return replace(QByteArrayView{copy}, after);
+        pinnedNeedle.assign(b, b + bsize);
+        b = pinnedNeedle.data();
     }
 
     QByteArrayMatcher matcher(b, bsize);