Fixup for #7, #9

This is my fixup for issues #7 and #9.

The problem was: When resizing the table, there is a new table allocated
on the stack of the function ht_resize(), which is then passed to another
function ht_insert_he(). This function takes the members of the
hash_table type and uses them for inserting the values. As they need to
be rehashed, it wants to use the hashfunctions as well. But these were
not passed to the new hashtable object!

This patch fixes this issue.
Fix up ht_destroy() and ht_resize()

This is a patch for consistancy: When destroying a hashtable, all of its
members are set to either zero or NULL. This patch adds the NULLifying
of the hashfunction pointers.

As the ht_resize function makes use of the ht_destroy() function, it has
to reset the hashfunction pointers to the hashtable object which got
resized.
merged segfault fix