QByteArray: more GCC 12 -Werror=array-bound whack-a-mole

This one came up on a tsan build, but not on an asan/ubsan one!?!:

    qbytearray.cpp: In member function ‘QByteArray QByteArray::toBase64(Base64Options) const’:
    qbytearray.cpp:3884:42: error: array subscript 1 is outside array bounds of ‘const char [1]’ [-Werror=array-bounds]
     3884 |             chunk |= int(uchar(data()[i++])) << 8;
          |                                ~~~~~~~~~~^
    qbytearray.cpp:37:24: note: at offset 1 into object ‘QByteArray::_empty’ of size 1
       37 | Q_CONSTINIT const char QByteArray::_empty = '\0';
          |                        ^~~~~~~~~~
    cc1plus: all warnings being treated as errors

Fix, as so often, by caching the size().

The code in 5.15 is different, but similar, so picking all the way.

Task-number: QTBUG-103923
Change-Id: Iac30e964c8d7d367620d16db65ceeaade33ee6b4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit de005e7976e4b110d6b5f32a8bab5141713acb7b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 1219b3d86d..bb517e3a53 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -3870,19 +3870,21 @@ QByteArray QByteArray::toBase64(Base64Options options) const
     const char padchar = '=';
     qsizetype padlen = 0;
 
-    QByteArray tmp((size() + 2) / 3 * 4, Qt::Uninitialized);
+    const qsizetype sz = size();
+
+    QByteArray tmp((sz + 2) / 3 * 4, Qt::Uninitialized);
 
     qsizetype i = 0;
     char *out = tmp.data();
-    while (i < size()) {
+    while (i < sz) {
         // encode 3 bytes at a time
         int chunk = 0;
         chunk |= int(uchar(data()[i++])) << 16;
-        if (i == size()) {
+        if (i == sz) {
             padlen = 2;
         } else {
             chunk |= int(uchar(data()[i++])) << 8;
-            if (i == size())
+            if (i == sz)
                 padlen = 1;
             else
                 chunk |= int(uchar(data()[i++]));