QPROPERTY_TEST_COMPARISON_HELPER: don't write 1KiB of NULs

qsnprint() is unconditionally called to fill the buffer and is
guaranteed to NUL-terminate, so there's no point in filling the whole
1KiB stack buffer with NULs beforehand. Leave it uninitialized.

Amends 930e59b798.

Pick-to: 6.7 6.5 6.2
Change-Id: I7da859d609ad4492c9038cb3e14f2a721b2acbe4
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 47c2263e896408e733c9aa466aec388cc4b4a6ad)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qpropertytesthelper_p.h

@@ -45,7 +45,7 @@ namespace QTestPrivate {
 #define QPROPERTY_TEST_COMPARISON_HELPER(actual, expected, comparator, represent)                  \
     do {                                                                                           \
         const size_t maxMsgLen = 1024;                                                             \
-        char msg[maxMsgLen] = { '\0' };                                                            \
+        char msg[maxMsgLen];                                                                       \
         auto actualStr = represent(actual);                                                        \
         auto expectedStr = represent(expected);                                                    \
         const size_t len1 = mbstowcs(nullptr, #actual, maxMsgLen);                                 \