QTextStream: remove a use of QString::sprintf()

Instead of using QString::sprintf() (and converting the result back to QByteArray),
simply do the conversion from uchar to hex digits ourselves, using QtMiscUtils.

This function is a copy of a similar (but not identical) one
in qprocess_unix.cpp, but it's not clear whether they can or
should be merged (both are only conditionally compiled), so
this patch does not attempt to do so.

Change-Id: I0be87963f78a98e35a54c98c5fb444756c57b672
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/io/qtextstream.cpp

@@ -233,6 +233,7 @@ static const int QTEXTSTREAM_BUFFERSIZE = 16384;
 
 #if defined QTEXTSTREAM_DEBUG
 #include <ctype.h>
+#include "private/qtools_p.h"
 
 QT_BEGIN_NAMESPACE
 
@@ -250,10 +251,16 @@ static QByteArray qt_prettyDebug(const char *data, int len, int maxSize)
         case '\n': out += "\\n"; break;
         case '\r': out += "\\r"; break;
         case '\t': out += "\\t"; break;
-        default:
-            QString tmp;
-            tmp.sprintf("\\x%x", (unsigned int)(unsigned char)c);
-            out += tmp.toLatin1();
+        default: {
+            const char buf[] = {
+                '\\',
+                'x',
+                QtMiscUtils::toHexLower(uchar(c) / 16),
+                QtMiscUtils::toHexLower(uchar(c) % 16),
+                0
+            };
+            out += buf;
+            }
         }
     }