Correctly process wholly translucent zvbi subtitles

A completely translucent teletext subtitle is sent to clear the old
subtitle, unfortunately my zvbi patch (f520484d) messed that use case
up, this one fixes it.

Thanks to Andre De Deudwaerder for finding the problem and tracking it down.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/zvbi.c

@@ -399,9 +399,13 @@ static subpicture_t *Decode( decoder_t *p_dec, block_t **pp_block )
 
     /* Ignore transparent rows at the beginning and end */
     int i_first_row = get_first_visible_row( p_page.text, p_page.rows, p_page.columns );
-    if ( i_first_row < 0 )
-        goto error;
-    int i_num_rows = get_last_visible_row( p_page.text, p_page.rows, p_page.columns ) - i_first_row + 1;
+    int i_num_rows;
+    if ( i_first_row < 0 ) {
+        i_first_row = p_page.rows - 1;
+        i_num_rows = 0;
+    } else {
+        i_num_rows = get_last_visible_row( p_page.text, p_page.rows, p_page.columns ) - i_first_row + 1;
+    }
 #ifdef ZVBI_DEBUG
     msg_Dbg( p_dec, "After top and tail of page we have rows %i-%i of %i",
              i_first_row + 1, i_first_row + i_num_rows, p_page.rows );
@@ -598,7 +602,7 @@ static int get_first_visible_row( vbi_char *p_text, int rows, int columns)
         }
     }
 
-    return rows;
+    return -1;
 }
 
 static int get_last_visible_row( vbi_char *p_text, int rows, int columns)
@@ -611,7 +615,7 @@ static int get_last_visible_row( vbi_char *p_text, int rows, int columns)
         }
     }
 
-    return 0;
+    return -1;
 }
 
 static int OpaquePage( picture_t *p_src, const vbi_page p_page,