For some long filenames the USTAR-split code does not work
correctly. It is wrongly assumed that the path would not be too long,
but it is.

The user visible result was that a filename was split, but it still
caused an error.

The cause was a wrongly calculated nlen. In addition I noticed that
at this place it is also seems necessary to check if the prefix will
fit in the 155 chars available for the prefix.

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/13300046