Fix clang static analyzer warnings.

Fix the following warning:
frameworks/av/media/mtp/MtpServer.cpp:1098:5: warning: Assigned value is
garbage or undefined [clang-analyzer-core.uninitialized.Assign]

BUG:None
TEST:Warnings fixed.
Change-Id: If4e54ebd9b865e2e6cdc7666da0c9987a918df7c