remove obsolete NULL casts

In any modern dialect of C, casting the "NULL" pointer literal to a
specific pointer type is unnecessary. For example:

char *foo;
foo = malloc(...);
if (foo == (char *) NULL) {...}

The cast on the 3rd line serves no useful purpose. Hence, this patch
removes all such instances of NULL-pointer casting from the
backend. I've attached it in gzip'ed format, as it is 145KB
uncompressed.

Unless anyone objects, I intend to apply this within 48 hours.

-Neil