Fix potential memleak (CID 260)

(cherry picked from commit cb89f6d5)
Signed-off-by: Derk-Jan Hartman <hartman@videolan.org>

modules/demux/subtitle.c

@@ -1898,7 +1898,10 @@ static int ParseRealText( demux_t *p_demux, subtitle_t *p_subtitle, int i_idx )
         const char *s = TextGetLine( txt );
 
         if( !s )
+        {
+            free( psz_text );
             return VLC_EGENERIC;
+        }
 
         int i_len = strlen( s );
         if( i_len == 0 ) break;