adb: fix undefined behavior

system/core/adb/shell_service_protocol_test.cpp:94:14: warning: Null
passed to a callee that requires a non-null 1st parameter
            !memcmp(data, protocol->data(), data_length));

C99 requires memcmp() to take non-null pointers.

Bug: none
Test: clang-tidy doesn't complain
Change-Id: I77b8d9373fa257a070ffc7fd318231c2c9ea84c3