bootm: bound-check OS index in bootm_os_get_boot_func()

bootm: bound-check OS index in bootm_os_get_boot_func()

The boot_os[] table in bootm_os.c is a sparse array whose compile-time
size is set by its largest designated initializer (IH_OS_ELF), giving
it IH_OS_ELF + 1 entries. The accessor bootm_os_get_boot_func() returns
boot_os[os] without any bound check, even though the caller in
bootm_run_states() passes images->os.os straight through. That field is
populated by image_get_os() from the raw 8-bit ih_os byte of a legacy
uImage, and by fit_image_get_os() for a FIT, neither of which clamps
the value against the table size.

An attacker-supplied image whose OS field falls outside the populated
range therefore drives an out-of-bounds read of boot_os[]. The caller
only rejects a NULL return, so a non-NULL adjacent global is accepted
as a valid handler and invoked through the indirect call in
boot_selected_os(), turning an unsigned image with a malformed header
into a jump through an attacker-influenced function pointer. FIT
signature verification covers the os property and mitigates this path
for signed images, but legacy bootm and unsigned FIT do not.

Reject out-of-range indices in bootm_os_get_boot_func() so the existing
NULL handling in bootm_run_states() reports an unsupported OS and
declines to boot the image.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>