library: fix a flaw in one 'new' function, <diskstats>

At 'new' time, the major API modules each ensure their
'info' parameter isn't NULL but what it pointed to was
except this single straggler, for some unknown reason.

So, this patch brings him into line with those others.

[ And, without going into the ugly details, this was ]
[ the reason I never experienced an abend originally ]
[ but Craig did. And, though related to stacks mgmt, ]
[ zero initialization was not a factor. Anyway, with ]
[ this patch, everybody would have experienced abend ]
[ under the original (faulty) test_Itemtables logic! ]

Reference(s):
https://www.freelists.org/post/procps/keep-on-patchin,13

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/diskstats.c

@@ -733,7 +733,7 @@ PROCPS_EXPORT int procps_diskstats_new (
     if (failed) _Exit(EXIT_FAILURE);
 #endif
 
-    if (info == NULL)
+    if (info == NULL || *info != NULL)
         return -EINVAL;
     if (!(p = calloc(1, sizeof(struct diskstats_info))))
         return -ENOMEM;