[libc] fix strncpy to zero out remaining space in the target
[libc] fix strncpy to zero out remaining space in the target

littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,travisg/lk,littlekernel/lk,littlekernel/lk,travisg/lk,travisg/lk,littlekernel/lk,travisg/lk,hollanderic/lkstuff,hollanderic/lkstuff