Initialise size of emulated instr in drx-scathergather.dll.c

I encountered a new flaky test on Travis:

https://travis-ci.com/github/DynamoRIO/dynamorio/jobs/331126794

194: Test command: /home/travis/build/DynamoRIO/dynamorio/build_debug-internal-32/bin32/drrun "-s" "90" "-quiet" "-debug" "-killpg" "-stderr_mask" "0xC" "-dumpcore_mask" "0" "-code_api" "-c" "/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-32/suite/tests/bin/libclient.drwrap-test.dll.so" "--" "/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-32/suite/tests/bin/client.drwrap-test" "/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-32/suite/tests/bin/libclient.drwrap-test.appdll.so"
194: Test timeout computed to be: 600
193: CHECK failed /home/travis/build/DynamoRIO/dynamorio/suite/tests/client-interface/drx-scattergather.dll.c:111: drmgr_get_emulated_instr_data() failed
194/335 Test #193: code_api|client.drx-scattergather ................................***Failed  Required regular expression not found.Regex=[^AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 gather ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX-512 scatter ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
AVX2 gather ok
Test restoring the AVX-512 gather scratch mask register upon a fault
Test restoring the AVX-512 scatter scratch mask register upon a fault
Test restoring the AVX-512 gather mask register upon translation events
Test restoring the AVX-512 scatter mask register upon translation events
Test updating the AVX-512 gather mask register upon translation events
Test updating the AVX-512 scatter mask register upon translation events
Test updating the AVX2 gather mask register upon translation events
AVX2/AVX-512 scatter/gather checks ok
event_exit, 23 scatter/gather instructions
$
]  0.84 sec
CHECK failed /home/travis/build/DynamoRIO/dynamorio/suite/tests/client-interface/drx-scattergather.dll.c:111: drmgr_get_emulated_instr_data() failed

drmgr_get_emulated_instr_data() returns false if the supplied emulated_instr does not have a sufficient size. Seems like the test, drx-scathergather.dll.c, forgot to do the initialization:

     emulated_instr_t emulated_instr;
            CHECK(drmgr_get_emulated_instr_data(instr, &emulated_instr),
                  "drmgr_get_emulated_instr_data() failed");