ChecksumParser: increment checksum_xref_id BEFORE use

The initial value of the variable $counter is set to the highest
checksum_xref.checksum_xref_id found if the table in question is not
empty, or 0 if it is. This causes problems if $counter is only
incremented after each use in the input-file loop:
 - for a non-empty table the parser would attempt to re-use an existing
   value of checksum_xref_id for the first entry read from the input
   file. checksum_xref_id is the primary key of checksum_xref so its
   values have to be unique, therefore "LOAD DATA" silently discards the
   offending row;
 - for an empty table we lose one input row as well but it is the SECOND
   rather than the first one. Reason: 0 is not a valid value for
   auto_increment fields in MySQL, resulting in the first row being
   inserted with the first allowed ID value of 1 - which brings us back
   to the previous scenario when "LOAD DATA" attempts to insert the
   second row.

Incrementing $counter before use ought to address both forms of the
problem.

misc-scripts/xref_mapping/XrefParser/ChecksumParser.pm

@@ -71,7 +71,13 @@ sub _transfer_contents {
   while(my $line = <$input_fh>) {
     chomp $line;
     my ($upi, $checksum) = split(/\s+/, $line);
-    my @output = ($counter++, $source_id, $upi, $checksum);
+
+    # Use an ID one higher than the last. Obvious? Perhaps - except before
+    # the commit adding this comment the code only incremented $counter
+    # AFTER using it.
+    $counter += 1;
+
+    my @output = ($counter, $source_id, $upi, $checksum);
     print $output_fh join("\t", @output);
     print $output_fh "\n";
   }