softmmu/physmem: Silence GCC 10 maybe-uninitialized error

When building with GCC 10.2 configured with --extra-cflags=-Os, we get:

  softmmu/physmem.c: In function ‘address_space_translate_for_iotlb’:
  softmmu/physmem.c:643:26: error: ‘notifier’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
    643 |         notifier->active = true;
        |                          ^
  softmmu/physmem.c:608:23: note: ‘notifier’ was declared here
    608 |     TCGIOMMUNotifier *notifier;
        |                       ^~~~~~~~

Initialize 'notifier' to silence the warning.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
v2: Remove pointless assert (Peter Maydell)

Yet another hole in our CI.

Supersedes: <20210117160754.4086411-1-f4bug@amsat.org>
69 jobs for meson_less in 71 minutes and 51 seconds (queued for 2 seconds)
latest
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  Build
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  Test
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  Deploy
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