lib: -Wformat-truncation in rsa_engine_get_priv_key

With glibc 2.33 (Ubuntu package glibc6 2.33-0ubuntu9) building
sifive_unmatched_defconfig results in:

In file included from /usr/include/stdio.h:866,
                  from ././include/compiler.h:26,
                  from <command-line>:
In function ‘snprintf’,
     inlined from ‘rsa_engine_get_priv_key’ at ./tools/../^:273:4:
/usr/include/riscv64-linux-gnu/bits/stdio2.h:71:10: warning:
‘%s’ directive argument is null [-Wformat-truncation=]
    71 |   return __builtin___snprintf_chk (__s, __n,
__USE_FORTIFY_LEVEL - 1,
       |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    72 |        __glibc_objsize (__s), __fmt,
       |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    73 |        __va_arg_pack ());
       |        ~~~~~~~~~~~~~~~~~

Avoid passing a NULL string.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>