Fix QStringConverter::encodingForName() for trailing -, _

The (internal) docs say that - and _ are ignored, and they're ignored
everywhere, except as suffixes. If the old code only ignored them as
infixes, fine, that would make some sense, but it ignored infixes and
prefixes, so there's no reason for it to not ignore suffixes, too.

Fix by continuing the loop until both input ranges are exhausted, or a
mismatch was found.

[ChangeLog][QtCore][QStringConverter] Fixed a bug where
encodingForName() failed due to trailing characters (`_`, `-`) that
ought to have been ignored.

Pick-to: 6.5
Change-Id: Iec21489d988eda7d33c744c170f88cd665b73f34
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 5f775e671973b1549a48ae8c69c5db7494f6a6d3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstringconverter.cpp

@@ -1761,21 +1761,16 @@ const QStringConverter::Interface QStringConverter::encodingInterfaces[QStringCo
 // match names case insensitive and skipping '-' and '_'
 static bool nameMatch(const char *a, const char *b)
 {
-    while (*a && *b) {
-        if (*a == '-' || *a == '_') {
+    do {
+        while (*a == '-' || *a == '_')
             ++a;
-            continue;
-        }
-        if (*b == '-' || *b == '_') {
+        while (*b == '-' || *b == '_')
             ++b;
-            continue;
-        }
-        if (QtMiscUtils::toAsciiLower(*a) != QtMiscUtils::toAsciiLower(*b))
-            return false;
-        ++a;
-        ++b;
-    }
-    return !*a && !*b;
+        if (!*a && !*b) // end of both strings
+            return true;
+    } while (QtMiscUtils::toAsciiLower(*a++) == QtMiscUtils::toAsciiLower(*b++));
+
+    return false;
 }
 
 

tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp

@@ -2238,6 +2238,8 @@ void tst_QStringConverter::encodingForName_data()
     row("ISO8859-1",   QStringConverter::Latin1);
     row("iso8859-1",   QStringConverter::Latin1);
     row("latin1",      QStringConverter::Latin1);
+    row("latin-1_-",   QStringConverter::Latin1);
+    row("latin_1-_",   QStringConverter::Latin1);
     row("-_latin-1",   QStringConverter::Latin1);
     row("_-latin_1",   QStringConverter::Latin1);