QIODevice: remove unneeded resize(1) in QIODevice::readLine()

AFAIU, the resize(1) was merely there to have result.size() return 1
in the initial iteration of the do loop. Since resize(1) is always
followed by another unconditional resize() in the do-while loop,
remove it and use "1 + readBytes" instead of result.size() in the
follow-up resize() call inside the do loop.

Saves one resize() call and the corresponding executable size and
runtime overhead.

Pick-to: 6.7 6.5
Change-Id: I507acab2a6b7d55217cd454ecbbaf1df0e1d18e6
Reviewed-by: Ivan Solovev <[email protected]>
Reviewed-by: Marc Mutz <[email protected]>
(cherry picked from commit d9579f0)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

src/corelib/io/qiodevice.cpp

@@ -1456,12 +1456,10 @@ QByteArray QIODevice::readLine(qint64 maxSize)
         // Size is unknown, read incrementally.
         maxSize = QByteArray::maxSize() - 1;
 
-        // The first iteration needs to leave an extra byte for the terminating null
-        result.resize(1);
-
         qint64 readResult;
         do {
-            result.resize(qsizetype(qMin(maxSize, qint64(result.size() + d->buffer.chunkSize()))));
+            // +1 since d->readLine() actually _writes_ a terminating NUL (### why does it?)
+            result.resize(qsizetype(qMin(maxSize, 1 + readBytes + d->buffer.chunkSize())));
             readResult = d->readLine(result.data() + readBytes, result.size() - readBytes);
             if (readResult > 0 || readBytes == 0)
                 readBytes += readResult;