Fixed seeking in mpeg1 files with libmpeg2 decoder (close #2953).

mpeg1 files may have only one sequence header.

It can be backported to 1.0 only after being carefully tested.
(cherry picked from commit 4746e4bc)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/libmpeg2.c

@@ -389,8 +389,14 @@ static picture_t *DecodeBlock( decoder_t *p_dec, block_t **pp_block )
                 if( !p_pic )
                 {
                     Reset( p_dec );
-                    block_Release( p_block );
-                    return NULL;
+
+                    p_pic = DpbNewPicture( p_dec );
+                    if( !p_pic )
+                    {
+                        mpeg2_reset( p_sys->p_mpeg2dec, 1 );
+                        block_Release( p_block );
+                        return NULL;
+                    }
                 }
             }
 
@@ -608,7 +614,7 @@ static void Reset( decoder_t *p_dec )
     decoder_sys_t *p_sys = p_dec->p_sys;
 
     cc_Flush( &p_sys->cc );
-    mpeg2_reset( p_sys->p_mpeg2dec, p_sys->p_info->sequence != NULL );
+    mpeg2_reset( p_sys->p_mpeg2dec, 0 );
     DpbClean( p_dec );
 }