Initialize DataBuf with 0s => fix valgrind error

src/image.cpp

@@ -404,6 +404,7 @@ namespace Exiv2 {
                     throw Error(kerInvalidMalloc);
                 }
                 DataBuf  buf((long)allocate);  // allocate a buffer
+                std::memset(buf.pData_, 0, buf.size_);
                 std::memcpy(buf.pData_,dir.pData_+8,4);  // copy dir[8:11] into buffer (short strings)
                 const bool bOffsetIsPointer = count*size > 4;