fix "scripts/mod/modpost.c: fix memory leak"

Fix error introduced by 37ed19d5


("scripts/mod/modpost.c: fix memory leak").

 - don't kfree("")

 - fix one missed conversion

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Alexey Fomenko <ext-alexey.fomenko@nokia.com>
Cc: Trevor Keith <tsrk@tsrk.net>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>