adb: fix uninitialized variable.

Previously, we were initializing IoBlock::pending when submitting a read,
but leaving it uninitialized when doing a write, which would sometimes
result in an assertion firing when we received a previously allocated
block of memory that happened to have a nonzero value in pending.

Test: treehugger
Change-Id: I434c65f554ceed313fea9cfcc98788908f3ca8bc