More patches based on clang reports

[Marin Ramesa]

[PATCH 1/5] kern/slab.c: initialize optimal_embed
http://darnassus.sceen.net/~teythoon/qa/gnumach/scan-build/2013-11-15/report-cAaz8T.html#EndPath

[PATCH 2/5] kern/slab.c: initialize info_size
http://darnassus.sceen.net/~teythoon/qa/gnumach/scan-build/2013-11-15/report-sSmdAq.html#EndPath

[PATCH 3/5] kern/processor.c: check if threads is NULL
http://darnassus.sceen.net/~teythoon/qa/gnumach/scan-build/2013-11-15/report-cJGp3r.html#EndPath

[PATCH 4/5] vm/vm_fault.c: check if active_threads is NULL
http://darnassus.sceen.net/~teythoon/qa/gnumach/scan-build/2013-11-15/report-yS4ODC.html#EndPath

[PATCH 5/5] vm/vm_fault.c: check if active_threads is NULL
http://darnassus.sceen.net/~teythoon/qa/gnumach/scan-build/2013-11-15/report-6fGYO8.html#EndPath