int.h: fix NDM_INT_MAX_STRLEN to use upper boundary
int.h: fix NDM_INT_MAX_STRLEN to use upper boundary

C99 requires integer constant expression to contain only
"integer constants, enumeration constants, character constants, sizeof
expressions whose results are integer constants, and floating constants
that are the immediate operands of casts.
...
More latitude is permitted for constant expressions in initializers.
Such a constant expression shall be, or evaluate to, one of the
following (a) an arithmetic constant expression, (b) a null pointer
constant, (c) an address constant, or (d) an address constant for an
object type plus or minus an integer constant expression."

Due to this NDM_INT_MAX_STRLEN(t) can not be considered as
a compile-time constant expression and Clang compiler warns about VLAs:

src/json.c: In function 'ndm_json_print_value_':
src/json.c:2486:13: warning: stack protector not protecting local
variables: variable length buffer [-Wstack-protector]
 static void ndm_json_print_value_(
             ^

To prevent it NDM_INT_MAX_STRLEN simplified
to return an upper numerical boundary.

ndmsystems/libndm