nwrap: Fix strotoul checks for NSS_WRAPPER_MAX_HOSTENTS

nwrap: Fix strotoul checks for NSS_WRAPPER_MAX_HOSTENTS

The env and endptr pointers need to be dereferenced, but that is not
enough: we don't really want to regard an empty string (*env == '\0')
as a valid number.

Found by GCC 8.0.0 20170705 (experimental).

[2095/4103] Compiling lib/nss_wrapper/nss_wrapper.c
../lib/nss_wrapper/nss_wrapper.c: In function "nwrap_init":
../lib/nss_wrapper/nss_wrapper.c:1571:13: warning: comparison between pointer and zero character constant [-Wpointer-compare]
   if (((env != '\0') && (endptr == '\0')) ||
                ^~
                ../lib/nss_wrapper/nss_wrapper.c:1571:9: note: did you mean to dereference the pointer?
   if (((env != '\0') && (endptr == '\0')) ||
            ^
            ../lib/nss_wrapper/nss_wrapper.c:1571:33: warning: comparison between pointer and zero character constant [-Wpointer-compare]
   if (((env != '\0') && (endptr == '\0')) ||
                                    ^~
                                    ../lib/nss_wrapper/nss_wrapper.c:1571:26: note: did you mean to dereference the pointer?
   if (((env != '\0') && (endptr == '\0')) ||

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Oct 19 16:42:17 CEST 2017 on sn-devel-144