Fix memory allocation edgecases in kvm_argv(..)

• Don't leak nbufp on realloc failure in kvm_argv
• Catch malloc errors with bufp
• Set buflen last in the "buflen == 0" case to ensure that bufp/nbufp is properly reallocated on the next go around

Differential Revision: https://reviews.freebsd.org/D6051
MFC after: 1 week
Reviewed by: jhb, markj
Reported by: cppcheck
Sponsored by: EMC / Isilon Storage Division