pragma: fix NULL-pointer dereference on malformed --pragma input

[cherry530]

Fixes: #300

When --pragma is given with a facility but no opname (e.g.
`nasm --pragma 'str' file.asm`), nasm_get_word() returns NULL and sets the
tail pointer to NULL.  process_pragma() then calls strlen() on the NULL
tail, causing a segmentation fault (NULL dereference).

Avoid the NULL dereference and report the malformed input explicitly:

    nasm --pragma 'str' t.asm
    t.asm: error: bad argument to %pragma str: missing opname

Verified: patch applies cleanly on master, full `./configure && make`
build succeeds (exit 0).