Added exception throw on Value pointer being null

v can be null if the typeId is invalid => throw an exception notifying
the user that his file is corrupted instead of the assertion

src/tiffvisitor.cpp

@@ -1554,7 +1554,9 @@ namespace Exiv2 {
             }
         }
         Value::AutoPtr v = Value::create(typeId);
-        assert(v.get());
+        if (!v.get()) {
+            throw Error(58);
+        }
         if ( !isize ) {
         	v->read(pData, size, byteOrder());
         } else {