In the functions invalidate_and_set_dirty() and
cpu_physical_memory_snapshot_and_clear_dirty(), we assume that we
are dealing with RAM memory regions. In this case we know that
memory_region_get_ram_addr() will succeed. Assert this before we
use the returned ram_addr_t in arithmetic.
This makes Coverity happier about these functions: it otherwise
complains that we might have an arithmetic overflow that stems
from the possible -1 return from memory_region_get_ram_addr().
Resolves: Coverity CID 1547629, 1547715
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
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