parallel: Fixed SQL racing error.

If two workers both work on the last job, the master will drop the
table when the first returns. The second should ignore the dropped
table.

src/parallel

@@ -10194,7 +10194,8 @@ sub get {
     my $self = shift;
     my $sth = $self->run(@_);
     my @retval;
-    while(1) {
+    # If $sth = 0 it means the table was dropped by another process
+    while($sth) {
 	my @row = $sth->fetchrow_array();
 	@row or last;
 	push @retval, \@row;