gitweb: check if HTTP_ACCEPT is really set

Signed-off-by: NMatthias Lederhofer <matled@gmx.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -856,7 +856,7 @@ sub git_header_html {
 	# 'application/xhtml+xml', otherwise send it as plain old 'text/html'.
 	# we have to do this because MSIE sometimes globs '*/*', pretending to
 	# support xhtml+xml but choking when it gets what it asked for.
-	if ($cgi->http('HTTP_ACCEPT') =~ m/(,|;|\s|^)application\/xhtml\+xml(,|;|\s|$)/ && $cgi->Accept('application/xhtml+xml') != 0) {
+	if (defined $cgi->http('HTTP_ACCEPT') && $cgi->http('HTTP_ACCEPT') =~ m/(,|;|\s|^)application\/xhtml\+xml(,|;|\s|$)/ && $cgi->Accept('application/xhtml+xml') != 0) {
 		$content_type = 'application/xhtml+xml';
 	} else {
 		$content_type = 'text/html';