incremental_server: fix a use of uninitalized memory

Without this, the caller is likely to assume that their buffer is
fully usable, which clang's analyzer doesn't believe is the case.

Another option is to set `*size` to nonzero.

Caught by the static analyzer:
system/core/adb/client/incremental_server.cpp:111:31: warning: 1st
function call argument is an uninitialized value
[clang-analyzer-core.CallAndMessage]

Bug: http://b/150032044
Test: TreeHugger
Change-Id: Ib844aa4ab3ebb297ca8f6f4289bbe3212275275b
Merged-In: Ib844aa4ab3ebb297ca8f6f4289bbe3212275275b
(cherry picked from commit 19b500bd)