Markdown.pl: ignore likely non-tag tags

Certain start tags (a, area, img, map) do not make sense unless
they have at least one attribute present.

If a completely attribute barren start tag for one of these elements
is found, treat it as invalid and escape the leading '<'.

This is an heuristic that shouldn't cause too many problems while
silently "correcting" incorrect input.

Either way (leaving the bare start tag with no attributes or escaping
it and potentially causing a fault as its end tag no longer has
anything to match up with), it's broken.

The question becomes then which breakage is more common in order
to handle that one in preference to the other.

With this change, the "it wasn't really a tag after all" situation
will now be considered more common than the "it was deliberatly an
invalid start tag with a matching end tag" situation.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -2251,6 +2251,7 @@ my %tagmt;	# empty element tags
 my %tagocl;	# non-empty elements with optional closing tag
 my %tagacl;	# which %tagocl an opening %tagocl will close
 my %tagblk;	# block elements
+my %taga1p;	# open tags which require at least one attribute
 my %lcattval;	# names of attribute values to lowercase
 my %impatt;	# names of "implied" attributes
 BEGIN {
@@ -2312,6 +2313,7 @@ BEGIN {
 	compact coords height hspace ismap nohref noshade nowrap rowspan size
 	span shape valign vspace width
     ));
+    %taga1p = map({$_ => 1} qw(a area img map));
 }
 
 
@@ -2493,8 +2495,9 @@ sub _Sanitize {
 	    $out .= "</$tt>" and $typ = 3 if $tag =~ m,/>$,;
 	}
 	return ($out,$typ);
-    } elsif ($tag =~ /^<([^\s<\/>]+)/gs) {
+    } elsif ($tag =~ /^<([^\s<\/>]+)/s) {
 	my $tt = lc($1);
+	return ("&lt;" . substr($tag,1), 0) if $taga1p{$tt};
 	if ($tagmt{$tt}) {
 	    return ("<" . $tt . $opt{empty_element_suffix}, 3);
 	} elsif ($tag =~ m,/>$,) {