glBufferSubData is calling vkMapMemory with a size of zero is invalid.
Check for that and exit early if found.

Bug: angleproject:2790

Change-Id: I965badeb3aa2cec1adc24dd7ff5695f8aa3e553d
Reviewed-on: https://chromium-review.googlesource.com/1194610
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>