preproc.c: Fix NULL deref on token pasting
preproc.c: Fix NULL deref on token pasting

In case if there is a whitespace before
'paste' token we may reach NULL dereference
in strlen since paste_head will point to
TOK_WHITESPACE. Fix it.

[mainstream adabc1576b957fc7d929d2c7e749b4f45f7293aa]

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@gmail.com>

letolabs/nasm,Distrotech/nasm,projedi/nasm,turingstudio/nasm,turingstudio/nasm,projedi/nasm,Distrotech/nasm,letolabs/nasm,letolabs/nasm,turingstudio/nasm,Distrotech/nasm,turingstudio/nasm,projedi/nasm,techkey/nasm,techkey/nasm,techkey/nasm,Distrotech/nasm,projedi/nasm,techkey/nasm,techkey/nasm,projedi/nasm