perf intel-pt: Fix snprintf size tracking bug in insn decoder

perf intel-pt: Fix snprintf size tracking bug in insn decoder

dump_insn() tracks remaining buffer space with a 'left' variable,
but the loop subtracts the cumulative offset 'n' each iteration
instead of just the per-iteration delta:

    n += snprintf(x->out + n, left, "%02x ", inbuf[i]);
    left -= n;  /* BUG: n is cumulative, not the delta */

After two iterations left goes massively negative, wrapping to a
huge value when passed as size_t to snprintf(), disabling all bounds
checking for the rest of the loop.

Switch to scnprintf() accumulation using sizeof(x->out) - n as the
remaining space, which is always correct and eliminates the separate
'left' variable entirely.

Fixes: 48d02a1d5c137d36 ("perf script: Add 'brstackinsn' for branch stacks")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>