QNonContiguousByteDevice: reset() instead of seek() if possible

... because e.g. QHttpMultiPartIODevice does not implement seek at all
(QTBUG-30295), which would make resetting such a device fail.

(cherry picked from commit 804a067846cd90e3ca31f985d539c8d75d654d82)

Change-Id: I8ba1ef76cc6e7f5ebf0b39c9a75e3aea26980b82
Reviewed-by: Shane Kearns <[email protected]>

Author: phartmann

src/corelib/io/qnoncontiguousbytedevice.cpp

@@ -389,8 +389,8 @@ bool QNonContiguousByteDeviceIoDeviceImpl::reset()
 {
     if (resetDisabled)
         return false;
-
-    if (device->seek(initialPosition)) {
+    bool reset = (initialPosition == 0) ? device->reset() : device->seek(initialPosition);
+    if (reset) {
         eof = false; // assume eof is false, it will be true after a read has been attempted
         totalAdvancements = 0; //reset the progress counter
         if (currentReadBuffer) {