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: None
Test: TreeHugger
Change-Id: Ib844aa4ab3ebb297ca8f6f4289bbe3212275275b