QString::assign: Don't crash when passed an empty range

Pick-to: 6.7
Change-Id: I7c02abeb1bd8fa5a8609f163a5a722c2c236fc2b
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit aa2cfb58089dcf46d14606964e30d1c695b64393)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstring.h

@@ -520,10 +520,12 @@ public:
             return *this;
         } else if constexpr (QtPrivate::IsCompatibleChar8Type<V>::value) {
             assign_helper_char8(first, last);
+            if (d.constAllocatedCapacity())
                 d.data()[d.size] = u'\0';
             return *this;
         } else {
             d.assign(first, last, [](QChar ch) -> char16_t { return ch.unicode(); });
+            if (d.constAllocatedCapacity())
                 d.data()[d.size] = u'\0';
             return *this;
         }

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -3668,6 +3668,25 @@ void tst_QString::assign()
         QCOMPARE(str.size(), 3);
     }
     // QString &assign(InputIterator, InputIterator)
+    {
+        // This needs to be on its own to ensure we call them on empty str
+        QString str;
+
+        const char16_t c16[] = u"٩(⁎❛ᴗ❛⁎)۶ 🤷";
+        std::u16string c16str(c16);
+        str.assign(c16str.begin(), c16str.begin());
+        QCOMPARE(str.size(), 0);
+    }
+    {
+#ifndef QT_NO_CAST_FROM_ASCII
+        // This needs to be on its own to ensure we call them on empty str
+        QString str;
+        const char c8[] = "a©☻🂤"; // [1, 2, 3, 4] bytes in utf-8 code points
+        std::string c8str(c8);
+        str.assign(c8str.begin(), c8str.begin());
+        QCOMPARE(str.size(), 0);
+#endif
+    }
     {
         // Forward iterator versions
         QString str;