Fix overflow issue on parsing min-qint64 with its minus sign repeated

The call to std::from_chars() accepts a sign, but we've already dealt
with a sign, so that would be a second sign. Check the first character
after any prefix is in fact a digit (for the base in use). This is a
follow-up to commit 5644af6f8a.

Fixes: QTBUG-97521
Change-Id: I65fb144bf6a8430da90ec5f65088ca20e79bf02f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale_tools.cpp

@@ -437,6 +437,19 @@ static auto scanPrefix(const char *p, const char *stop, int base)
     return R{p, base};
 }
 
+static bool isDigitForBase(char d, int base)
+{
+    if (d < '0')
+        return false;
+    if (d - '0' < qMin(base, 10))
+        return true;
+    if (base > 10) {
+        d |= 0x20; // tolower
+        return d >= 'a' && d < 'a' + base - 10;
+    }
+    return false;
+}
+
 unsigned long long
 qstrntoull(const char *begin, qsizetype size, const char **endptr, int base, bool *ok)
 {
@@ -479,7 +492,9 @@ qstrntoll(const char *begin, qsizetype size, const char **endptr, int base, bool
         ++p;
 
     const auto prefix = scanPrefix(p, stop, base);
-    if (!prefix.base || prefix.next >= stop) {
+    // Must check for digit, as from_chars() will accept a sign, which would be
+    // a second sign, that we should reject.
+    if (!prefix.base || prefix.next >= stop || !isDigitForBase(*prefix.next, prefix.base)) {
         if (endptr)
             *endptr = begin;
         *ok = false;

tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp

@@ -2080,6 +2080,7 @@ void tst_QStringApiSymmetry::toNumber_data()
     QTest::addRow("-32768") << QString::fromUtf8("-32768") << qint64(-32768) << true;
     QTest::addRow("100x") << QString::fromUtf8("100x") << qint64(0) << false;
     QTest::addRow("-100x") << QString::fromUtf8("-100x") << qint64(0) << false;
+    QTest::addRow("-min64") << QString::fromUtf8("--9223372036854775808") << qint64(0) << false;
 }
 
 template<typename T>