Clean spatial picture alloc code

Remove useless code.

in the function AllocSpatialPictures: which need not to call FreeMemorySvc on memory alloc fail, it is handled well at the code where AllocSpatialPicture is called. pls see the line 867 @ encoder_ext.cpp. So we can use pCtx to replace ppCtx.

Review: 
https://rbcommons.com/s/OpenH264/r/78/