Fix #37221: Multilayer EXR inputs generate pink frame/last displayed frame when using an image sequence offset

generally speaking, if multilayer image fails to load for current
frame doesn't mean anything bad. It might be used to make it so
image sequence is being alpha-overed somewhere in the middle of
scene time.

Made it so if the whole file fails to load, image node will
deliver black transparent color, the same what happens for
regular (non-multilayer images).

Also needed to tweak code in load_multilayer_sequwnce to make
sure no cached frames are pointing to a freed memory.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60965 954f8c5b-7b00-dc11-b283-0030488c597c