lib: errno: avoid error format-overflow

lib: errno: avoid error format-overflow

In cmd/regulator.c an error occurs with GCC 9.2.1 if CONFIG_ERRNO_STR is
not defined:

cmd/regulator.c: In function ‘failure’:
cmd/regulator.c:20:2: error: ‘%s’ directive argument is null
[-Werror=format-overflow=]
   20 |  printf("Error: %d (%s)\n", ret, errno_str(ret));
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘constraint’,
    inlined from ‘constraint’ at cmd/regulator.c:111:12:
cmd/regulator.c:115:3: error: ‘%s’ directive argument is null
[-Werror=format-overflow=]
  115 |   printf(" %s (err: %d)\n", errno_str(val), val);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

errno_str() should return a valid string instead of NULL if
CONFIG_ERRNO_STR is not defined.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>