lib: sbi: check result of pmp_get() in is_pmp_entry_mapped()

pmp_get() may return an error if the given entry, given by the caller
of is_pmp_entry_mapped(), is invalid. This results in the output
parameters for pmp_get() being uninitialized. To avoid using garbage
values, check the result and return early if necessary.

This issue is not being hit because at the moment
is_pmp_entry_mapped() is only being called from a single site with a
valid hardcoded value.

Signed-off-by: Carlos López <carlos.lopezr4096@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>