Added assertion that call to vsnprintf succeeds

src/image.cpp

@@ -1004,6 +1004,7 @@ namespace Exiv2 {
             va_start(args, format);                        // args start after format
             rc = vsnprintf(&buffer[0], buffer.size(), format, args);
             va_end(args);                                  // free the args
+            assert(rc >= 0);                               // rc < 0 => we have made an error in the format string
             if ( rc > 0 )
                 need = static_cast<size_t>(rc);
         } while ( buffer.size() <= need );