syscalls can return negative i/o counts. fix bugs in ReadAt and WriteAt not to include negative counts in return values.
syscalls can return negative i/o counts. fix bugs in ReadAt and WriteAt not to include
negative counts in return values.

R=rsc
CC=golang-dev
http://codereview.appspot.com/170044

gnanderson/go,tianon/debian-golang,bdwalton/debian-golang,dsriram/golang,ssrl/go,tv42/old-go,tv42/old-go,apparentlymart/golang,h8liu/golang,dsriram/golang,gnanderson/go,dsriram/golang,h8liu/golang,ssrl/go,tianon/debian-golang,tv42/old-go,pombredanne/go-deleteme,tianon/debian-golang,nightlyone/golang,apparentlymart/golang,bdwalton/debian-golang,pombredanne/go-deleteme,ssrl/go,h8liu/golang,bdwalton/debian-golang,h8liu/golang,nightlyone/golang,apparentlymart/golang,nightlyone/golang,gnanderson/go,ssrl/go,tv42/old-go,bdwalton/debian-golang,nightlyone/golang,tv42/old-go,nightlyone/golang,pombredanne/go-deleteme,apparentlymart/golang,bdwalton/debian-golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tianon/debian-golang,nightlyone/golang,ssrl/go,dsriram/golang,bdwalton/debian-golang,bdwalton/debian-golang,apparentlymart/golang,gnanderson/go,dsriram/golang,h8liu/golang,tianon/debian-golang,gnanderson/go,bdwalton/debian-golang,tianon/debian-golang,ssrl/go,tianon/debian-golang,dsriram/golang,tv42/old-go,apparentlymart/golang,h8liu/golang,tianon/debian-golang,pombredanne/go-deleteme,pombredanne/go-deleteme,ssrl/go,nightlyone/golang,h8liu/golang,gnanderson/go,gnanderson/go,dsriram/golang