Fix size types

This fixes compilation with -Wstrict-overflow=2

with CFLAGS="-O3 -Werror=strict-overflow -Wstrict-overflow=2"
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.10) gives:

config_file.c: In function ‘krb5_config_vget_strings’:
config_file.c:1122:10: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow]
     while(nstr--)
          ^

Upstream pull request:
https://github.com/heimdal/heimdal/pull/354

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(similar to Samba commit 72979d1d60ca2eab1e7903c2e77b8cca69667691,
cut down to just the config_file.c and keytab_any.c changes reproduced
above by abartlet)

lib/krb5/config_file.c

@@ -1081,7 +1081,7 @@ krb5_config_vget_strings(krb5_context context,
 			 va_list args)
 {
     char **strings = NULL;
-    int nstr = 0;
+    size_t nstr = 0;
     const krb5_config_binding *b = NULL;
     const char *p;