qppmhandler: Fix undefined overflow behavior

image/qppmhandler.cpp:77:25: runtime error: signed integer overflow: 10 * 300000000 cannot be represented in type 'int'
    #0 0x4cecb5 in read_pbm_int(QIODevice*) /src/qt/qtbase/src/gui/image/qppmhandler.cpp:77:25
    #1 0x4cb1ac in read_pbm_body(QIODevice*, char, int, int, int, QImage*) /src/qt/qtbase/src/gui/image/qppmhandler.cpp:271:29
    #2 0x4ca3d8 in QPpmHandler::read(QImage*) /src/qt/qtbase/src/gui/image/qppmhandler.cpp:509:10
    #3 0x4b238d in QImageReader::read(QImage*) /src/qt/qtbase/src/gui/image/qimagereader.cpp:1253:22
    #4 0x4b1b61 in QImageReader::read() /src/qt/qtbase/src/gui/image/qimagereader.cpp:1201:12
    #5 0x486f66 in QImage::fromData(unsigned char const*, int, char const*) /src/qt/qtbase/src/gui/image/qimage.cpp:3624:37
    #6 0x486cd8 in QImage::loadFromData(unsigned char const*, int, char const*) /src/qt/qtbase/src/gui/image/qimage.cpp:3590:13
    #7 0x434b2e in LLVMFuzzerTestOneInput /src/qimage_fuzzer.cc:28:7
    #8 0x44b167 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:570:15
    #9 0x44a535 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:479:3
    #10 0x44c428 in fuzzer::Fuzzer::MutateAndTestOne() /src/libfuzzer/FuzzerLoop.cpp:707:19
    #11 0x44d1b5 in fuzzer::Fuzzer::Loop(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:838:5
    #12 0x440a29 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:764:6
    #13 0x434bf8 in main /src/libfuzzer/FuzzerMain.cpp:20:10
    #14 0x7fba939a082f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #15 0x409bb8 in _start (/out/qimage_fuzzer+0x409bb8)

Change-Id: I9ad78afc4ea9c5c8b7530aa17013abe91202e84b
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qppmhandler.cpp

@@ -68,13 +68,19 @@ static int read_pbm_int(QIODevice *d)
     char c;
     int          val = -1;
     bool  digit;
+    bool hasOverflow = false;
     for (;;) {
         if (!d->getChar(&c))                // end of file
             break;
         digit = isdigit((uchar) c);
         if (val != -1) {
             if (digit) {
-                val = 10*val + c - '0';
+                const int cValue = c - '0';
+                if (val <= (INT_MAX - cValue) / 10) {
+                    val = 10*val + cValue;
+                } else {
+                    hasOverflow = true;
+                }
                 continue;
             } else {
                 if (c == '#')                        // comment
@@ -91,7 +97,7 @@ static int read_pbm_int(QIODevice *d)
         else
             break;
     }
-    return val;
+    return hasOverflow ? -1 : val;
 }
 
 static bool read_pbm_header(QIODevice *device, char& type, int& w, int& h, int& mcc)