Fix for CVE-2017-14860

A heap buffer overflow could occur in memcpy when icc.size_ is larger
than data.size_ - pad, as then memcpy would read out of bounds of data.

This commit adds a sanity check to iccLength (= icc.size_): if it is
larger than data.size_ - pad (i.e. an overflow would be caused) an
exception is thrown.

This fixes #71.

src/jp2image.cpp

@@ -269,10 +269,15 @@ namespace Exiv2
                             std::cout << "Exiv2::Jp2Image::readMetadata: "
                                      << "Color data found" << std::endl;
 #endif
-                            long pad = 3 ; // 3 padding bytes 2 0 0
+                            const long pad = 3 ; // 3 padding bytes 2 0 0
                             DataBuf data(subBox.length+8);
                             io_->read(data.pData_,data.size_);
-                            long    iccLength = getULong(data.pData_+pad, bigEndian);
+                            const long    iccLength = getULong(data.pData_+pad, bigEndian);
+                            // subtracting pad from data.size_ is safe:
+                            // size_ is at least 8 and pad = 3
+                            if (iccLength > data.size_ - pad) {
+                                throw Error(58);
+			    }
                             DataBuf icc(iccLength);
                             ::memcpy(icc.pData_,data.pData_+pad,icc.size_);
 #ifdef DEBUG