Break parsing if no processes are found

Break parsing if no processes are found

Scanning through all pids in the system can be a time consuming task,
especially with slower CPUs. Therefore, if the user gives a
nonexistent process argument it can tage significant amount of time
until the user gets the response that there will be no useful results
available.

This patch counts the number of processes that have been encoutered
during the prescan. If the count is zero, then it is no good to
proceed scanning all of the rest processes. As an extra benefit, the
number of prescanned processes can be used for something useful..

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>