sym(4): fix nits reported by Coverity

• In ___dma_getp(), remove dead code. [1]
• In sym_sir_bad_scsi_status(), add missing FALLTHROUGH. [2]

While at it:

• For getbaddrcb(), remove __unused from the nseg argument as it's in fact used when compiling with INVARIANTS.
• In sym_int_sir(), ensure in all branches that cp is not NULL before using it.

Reported by: Coverity
CID: 1008861 [1], 114996 [2]