backtrace_create_state_for_file opens the file lazily on first use and,
on a failed open, silently falls back to /proc/self/exe - so a state
created from a path the monitor cannot open would symbolize the target's
addresses against the monitor's own binary. Probe the target's root path
(and, for unlinked images, the mapping's map_files entry), and only
create a state for a path that actually opens.
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