Change NewDirectByteBuffer to allow NULL if capacity == 0.

Also make the diagnostics more uniform.

Bug: 7892060
Change-Id: Ib3ca0d6241a5958d2c8681337b1883fc3d74cb54
dalvik: remove duplicated call to dvmJitCalleeSave()

Change-Id: Ib7b5f637a22fd5ba60bdb45cd34ac8c1781c7bd3
Author: Rocky Zhang <zhangyan.hit@gmail.com>
Signed-off-by: Rocky Zhang <zhangyan.hit@gmail.com>