Use after free in ami(4)

[Arto Jonsson]

Spotted by LLVM static analyser.

In /usr/src/sys/dev/ic/ami.c line 499 ami_freemem(sc, am) is called. In
the next block ami_alloc_ccbs(...) is called. If the result != 0 control
jumps to free_mbox in line 614. This falls through to call
ami_freemem(sc, am) again.