fix check of return value of firstidx (-1 on not found, instead of undef)

Author: wchristian

lib/PPI/Node.pm

@@ -512,7 +512,7 @@ sub remove_child {
 	my $p   = List::MoreUtils::firstidx {
 		refaddr $_ == $key
 	} @{$self->{children}};
-	return undef unless defined $p;
+	return undef if $p == -1;
 
 	# Splice it out, and remove the child's parent entry
 	splice( @{$self->{children}}, $p, 1 );

t/ppi_node.t

@@ -42,6 +42,5 @@ REMOVE_CHILD: {
 	my $del1 = $node->child(7);
 	is $node->remove_child($del1), $del1;
 	my $fake = bless { content => 3 }, "PPI::Token::Number";
-	local $TODO = "firstidx is mis-used here";
 	is $node->remove_child($fake), undef;
 }