Fix bug in handling of child process exit.

When storage_mon detects that a child process exits with zero,
it resets the test_forks[] entry for the child process to 0, to avoid
waitpid() for the process again in the loop. But, previously,
storage_mon didn't do that when it detected that a child process
exited with non-zero. Which caused waitpid() to be called again
for the process already gone and to report an error like
"waitpid on XXX failed: No child processes" unexpectedly.
In this case, basically storage_mon should wait until all the child
processes exit and return the final score, instead.

This patch fixes this issue by making storage_mon reset test_works[]
entry even when a child process exits with non-zero.