gitweb: Check that $site_header etc. are defined before using them

gitweb: Check that $site_header etc. are defined before using them

If one of $site_header, $site_footer or $home_text is not defined, you
get extraneous errors in the web logs, for example (line wrapped for
better readibility):

 [Wed Jan 13 16:55:42 2010] [error] [client ::1] [Wed Jan 13 16:55:42 2010]
 gitweb.cgi: Use of uninitialized value $site_header in -f at
 /var/www/gitweb/gitweb.cgi line 3287., referer: http://git/gitweb.cgi

This ensures that those variables are defined before trying to use it.

Note that such error can happen only because of an error in gitweb
config file; building gitweb.cgi can make mentioned variables holding
empty string (it is even the default), but they are still defined.

Signed-off-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>