ASSERT signal.c:3728 decode() != NULL && instr_is_syscall()

This happened on a 3rd-party app using static DR and static drmemtrace (the same one that hit #2993 (closed)):

<Attached to 9 threads in application xxx (6316)>
<dropping 3rd pending alarm signal>
<Application xxx (6316).  Internal Error: DynamoRIO debug check failure: core/unix/signal.c:3728 decode(dcontext, (app_pc) sc->SC_XIP, &instr) != NULL && instr_is_syscall(&instr)
(Error occurred @7796 frags)