Fixed random word to use whole file.

Fixed random word to use whole file.
The getrandomword function did not use the whole file. Changed to seek
to a random location and then read the next full word that is 7 chars.
Checked for memleaks using valgrind and added cleanup for the word
tree and answers list. No more leaks reported.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>