input: stream_memory: handle skip reads

Regular streams skip if p_read is NULL.

(cherry picked from commit 426d82731aa74530af562c3fa09ce7a2eeb45c6a)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/input/stream_memory.c

@@ -157,7 +157,8 @@ static int Read( stream_t *s, void *p_read, unsigned int i_read )
 {
     stream_sys_t *p_sys = s->p_sys;
     int i_res = __MIN( i_read, p_sys->i_size - p_sys->i_pos );
-    memcpy( p_read, p_sys->p_buffer + p_sys->i_pos, i_res );
+    if ( p_read )
+        memcpy( p_read, p_sys->p_buffer + p_sys->i_pos, i_res );
     p_sys->i_pos += i_res;
     return i_res;
 }