Remember that undef has been found to avoid calling the coderef again

This is useful when working with coderefs that defer to
`$sth->fetchrow_arrayref`, which throws an error if called again after
having returned undef (which signifies the end of the data stream)

```
DBD::mysql::st fetchrow_arrayref failed: fetch() without execute()
```

modules/Bio/EnsEMBL/Utils/Iterator.pm

@@ -155,9 +155,12 @@ sub new {
 sub next {
     my $self = shift;
 
-    $self->{next} = $self->{sub}->() unless defined $self->{next};
-    
-    return delete $self->{next};
+    $self->{next} = $self->{sub}->() unless exists $self->{next};
+
+    if (defined $self->{next}) {
+        return delete $self->{next};
+    }
+    return;
 }
 
 =head2 has_next
@@ -175,7 +178,7 @@ sub next {
 sub has_next {
     my $self = shift;
 
-    $self->{next} = $self->{sub}->() unless defined $self->{next};
+    $self->{next} = $self->{sub}->() unless exists $self->{next};
 
     return defined $self->{next}; 
 }
@@ -196,7 +199,7 @@ sub has_next {
 sub peek {
     my $self = shift;
 
-    $self->{next} = $self->{sub}->() unless defined $self->{next};
+    $self->{next} = $self->{sub}->() unless exists $self->{next};
 
     return $self->{next};
 }

modules/t/iterator.t

@@ -33,6 +33,23 @@ $it = Bio::EnsEMBL::Utils::Iterator->new(sub {return 3});
 
 is($it->next, 3, "got expected value from iterator created from coderef");
 
+# check that the coderef is not called any more once it returns undef
+
+my $call_counter = 0;
+$it = Bio::EnsEMBL::Utils::Iterator->new(sub {
+        ++$call_counter;
+        if ($call_counter == 1) {
+            return 1;
+        } else {
+            return undef;
+        }
+    });
+
+is($it->next, 1, "got expected value from iterator created from coderef");
+is($it->next, undef, "got undef from iterator created from coderef once exhausted");
+is($it->next, undef, "got undef from iterator created from coderef once exhausted");
+is($call_counter, 2, 'Returned undef without calling the coderef any further');
+
 # from now on we create an iterator from an arrayref because it's simpler
 
 $it = Bio::EnsEMBL::Utils::Iterator->new([1,2,3]);