asm/preproc.c: use nasm_add_string_to_strlist()
asm/preproc.c: use nasm_add_string_to_strlist()

Use nasm_add_string_to_strlist() to avoid a memory leak.
nasm_add_to_strlist() requires that the caller manages the string
being added or not.

Signed-off-by: H. Peter Anvin <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm