change
printf("Decimal to base %d is :\t%s\n",base,conv);
to
printf("Decimal to base %d is :\t%s\n",base,ptr);
or
move the contents (ptr - to end of string) to start of conv array
another alternative is to instead of writing from end of conv
to write from start and then when you are done call strrev()
to reverse it.
printf("Decimal to base %d is :\t%s\n",base,strrev(conv));