altq: Stop checking for failures from malloc(M_WAITOK)

While here, prefer malloc(M_ZERO) over bzero().

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45852

(cherry picked from commit 2a119886630bb5fe8283e20db5acb7c0cdba31c3)
(cherry picked from commit 891fdd020a6826eb98d30284f78000e666f9a263)