Fix crash in tqimage for certain malformed ppm image files

The ppm format specifies that the maximum color value field must be
less than 65536. The handler did not enforce this, leading to
potentional overflow when the value was used in 16 bits context.

Based on Qt5 patch for CVE-2018-19872.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>

src/kernel/qimage.cpp

@@ -5196,7 +5196,7 @@ static void read_pbm_image( QImageIO *iio )	// read PBM image data
 	mcc = 1;				// ignore max color component
     else
 	mcc = read_pbm_int( d );		// get max color component
-    if ( w <= 0 || w > 32767 || h <= 0 || h > 32767 || mcc <= 0 )
+    if ( w <= 0 || w > 32767 || h <= 0 || h > 32767 || mcc <= 0 || mcc > 0xffff )
 	return;					// weird P.M image
 
     int maxc = mcc;