QPpmHandler crashes when malformed image causes dividing by zero (SIGFPE)

1. Created a short source code that uses QImage::load()
2. Fed the compiled program with a malformed .ppm image (mutated by AFL-fuzzer)
3. Assumed that the QImage would fail gracefully
4. SIGFPE crash instead

Stackstrace:

scale_pbm_color(unsigned short, unsigned short, unsigned short, unsigned short) (gv=..., bv=..., mx=..., rv=...) at image/qppmhandler.cpp:126
read_pbm_body(QIODevice*, char, int, int, int, QImage*) (device=..., w=..., h=..., mcc=..., outImage=..., type=...) at image/qppmhandler.cpp:193
QPpmHandler::read(QImage*) (this=..., image=...) at image/qppmhandler.cpp:509
QImageReader::read(QImage*) (this=..., image=...) at image/qimagereader.cpp:1253
QImageReader::read() (this=...) at image/qimagereader.cpp:1201
QImage::load(QString const&, char const*) (this=..., fileName=..., format=...) at image/qimage.cpp:3430
main(int, char**) (argc=..., argv=...) at main.cpp:15

Quick analysis:

scale_pbm_color expects unsigned short, but mcc that gets passed from read_pbm_body is int. Conversion from from int to unsigned short truncates the value to zero, which causes division by zero.

Possible fix:

Do a quick sanity check in read_pbm_body to ensure that the conversion is meaningful.