Fix memory allocation/copy mistakes.

The previous code was allocating more memory and copying more data than
necessary because it specified the wrong PgStat_KindInfo member as the
size argument for MemoryContextAlloc and memcpy, respectively.

Although these issues exist since 5891c7a8e, there have been no reports
from the field.  So for now, it seems sufficient to fix them in master.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Discussion: https://postgr.es/m/CAPmGK15eTRCZTnfgQ4EuBNo%3DQLYGFEbXS_7m2dXqtkcT7L8qrQ%40mail.gmail.com

src/backend/utils/activity/pgstat.c

@@ -1187,7 +1187,7 @@ pgstat_build_snapshot(void)
 		Assert(!found);
 
 		entry->data = MemoryContextAlloc(pgStatLocal.snapshot.context,
-										 kind_info->shared_size);
+										 pgstat_get_entry_len(kind));
 
 		/*
 		 * Acquire the LWLock directly instead of using
@@ -1196,7 +1196,7 @@ pgstat_build_snapshot(void)
 		LWLockAcquire(&stats_data->lock, LW_SHARED);
 		memcpy(entry->data,
 			   pgstat_get_entry_data(kind, stats_data),
-			   kind_info->shared_size);
+			   pgstat_get_entry_len(kind));
 		LWLockRelease(&stats_data->lock);
 	}
 	dshash_seq_term(&hstat);