Fix miserable coding in pg_stat_get_activity().

Commit dd1a3bcc replaced a test on whether a subroutine returned a
null pointer with a test on whether &pointer->backendStatus was null.
This accidentally failed to fail, at least on common compilers, because
backendStatus is the first field in the struct; but it was surely trouble
waiting to happen.  Commit f91feba8 then messed things up further,
changing the logic to

	local_beentry = pgstat_fetch_stat_local_beentry(curr_backend);
	if (!local_beentry)
		continue;
	beentry = &local_beentry->backendStatus;
	if (!beentry)
	{

where the second "if" is now dead code, so that the intended behavior of
printing a row with "<backend information not available>" cannot occur.

I suspect this is all moot because pgstat_fetch_stat_local_beentry
will never actually return null in this function's usage, but it's still
very poor coding.  Repair back to 9.4 where the original problem was
introduced.

src/backend/utils/adt/pgstatfuncs.c

@@ -688,27 +688,17 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 		MemSet(values, 0, sizeof(values));
 		MemSet(nulls, 0, sizeof(nulls));
 
-		if (pid != -1)
-		{
-			/* Skip any which are not the one we're looking for. */
-			PgBackendStatus *be = pgstat_fetch_stat_beentry(curr_backend);
-
-			if (!be || be->st_procpid != pid)
-				continue;
-
-		}
-
 		/* Get the next one in the list */
 		local_beentry = pgstat_fetch_stat_local_beentry(curr_backend);
 		if (!local_beentry)
-			continue;
-
-		beentry = &local_beentry->backendStatus;
-		if (!beentry)
 		{
 			int			i;
 
-			for (i = 0; i < sizeof(nulls) / sizeof(nulls[0]); i++)
+			/* Ignore missing entries if looking for specific PID */
+			if (pid != -1)
+				continue;
+
+			for (i = 0; i < lengthof(nulls); i++)
 				nulls[i] = true;
 
 			nulls[5] = false;
@@ -718,6 +708,12 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 			continue;
 		}
 
+		beentry = &local_beentry->backendStatus;
+
+		/* If looking for specific PID, ignore all the others */
+		if (pid != -1 && beentry->st_procpid != pid)
+			continue;
+
 		/* Values available to all callers */
 		values[0] = ObjectIdGetDatum(beentry->st_databaseid);
 		values[1] = Int32GetDatum(beentry->st_procpid);