() Write a C function, copy_append_string () that takes a two strings, s and t, possibly NULL, as its parameters and returns a pointer to a newly allocated region of memory containing a string that is the concatenation of the two. (That is, copy_append_string("Hi ", "There") would return a pointer to "Hi There"). Write robust code. That is, return NULL on failure, but do not crash. Think before you write anything. char *copy_append_string ( char s, char t){