Fix rendering of replacement_char when Charset_is_utf8

Use mutt_addwch instead of addch to draw the replacement_char in
pager.c. The line to draw the replacement char was
addch (replacement_char ())
However, if Charset_is_utf8 is true, replacement_char returns 0xfffd,
which is outside the range that addch can handle. Use mutt_addwch
instead so that the pager displays the replacement char properly.

pager.c

···
 	break;
       ++col;
       if (pa)
-	addch (replacement_char ());
+	mutt_addwch (replacement_char ());
     }
   }
   *pspace = space;