Merge pull request #107 from D4N/tiffvisitor_nullptr_check

Use nullptr check instead of assertion, by Raphaël Hertzog

src/tiffvisitor.cpp

@@ -1294,11 +1294,12 @@ namespace Exiv2 {
             }
             uint16_t tag = getUShort(p, byteOrder());
             TiffComponent::AutoPtr tc = TiffCreator::create(tag, object->group());
-            // The assertion typically fails if a component is not configured in
-            // the TIFF structure table
-            assert(tc.get());
+            if (tc.get()) {
                 tc->setStart(p);
                 object->addChild(tc);
+            } else {
+               EXV_WARNING << "Unable to handle tag " << tag << ".\n";
+            }
             p += 12;
         }