SIGSEGV loop in signal handler when DR is setup but there's no thread-private dcontext

I discovered this when investigating a segfault in drcachesim (https://github.com/DynamoRIO/dynamorio/issues/2663). If there is a segfault in normal application code after DR's signal handler has been installed and when there's no specific `dcontext` set for that thread, the signal handler itself will segfault. This causes an infinite segfault loop.

When `get_thread_private_dcontext` returns NULL, the `dcontext` is set to `GLOBAL_CONTEXT` [0]. Later in the function, a field on the `dcontext` is accessed [1], which causes a segfault and causes the signal handler to be called again.

[0] https://github.com/DynamoRIO/dynamorio/blob/35a6c54/core/unix/signal.c#L4542
[1] https://github.com/DynamoRIO/dynamorio/blob/35a6c54/core/unix/signal.c#L4688