[Coverity 1665660] Dereference null return value in stats.c

If the function actually returns a null value, a null pointer dereference will occur.

In EncodeVideo: Return value of function which returns null is dereferenced without checking (CWE-476)

      1. returned_null: vlc_frame_Alloc returns NULL (checked 198 out of 235 times).[show details]
      2. var_assigned: Assigning: p_block = NULL return value from vlc_frame_Alloc.
104    block_t * p_block = block_Alloc( kBufferSize );
105
106    vlc_tick_t now = vlc_tick_now();
     
CID 1665660: (#1 of 1): Dereference null return value (NULL_RETURNS)
3. dereference: Dereferencing p_block, which is known to be NULL.
107    memcpy(p_block->p_buffer, &now, sizeof(vlc_tick_t));