scripts/kallsyms: fix memory corruption caused by write over-run

memcpy() writes one more byte than allocated.

Fixes: 8d605269 ("scripts/kallsyms: change table to store (strcut sym_entry *)")
Reported-by: youling257 <youling257@gmail.com>
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Pavel Machek <pavel@ucw.cz>