[binaryToString] Fixed overread when passing start > 0

binaryToString(DataBuf&) only calls the C-array version. Unfortunately, its
safety check is flawed, as it only works when start is 0 (probably due to its
conterintuitive interface).
binaryToString(byte*) formats the range (start,start+size) => we have to trimm
size if size + start is larger than the DataBuf's size_

src/image_int.cpp

@@ -80,8 +80,9 @@ namespace Exiv2
 
         std::string binaryToString(const DataBuf& buf, size_t size, size_t start /*=0*/)
         {
-            if (size > (size_t)buf.size_)
-                size = (size_t)buf.size_;
+            if (size + start > static_cast<size_t>(buf.size_)) {
+                size = static_cast<size_t>(buf.size_) - start;
+            }
             return binaryToString(buf.pData_, size, start);
         }