Fix compiler warning

from gcc-7 -Wformat-truncation (via -Wall)

src/bin/initdb/initdb.c

@@ -1009,12 +1009,12 @@ static char *
 pretty_wal_size(int segment_count)
 {
 	int			sz = wal_segment_size_mb * segment_count;
-	char	   *result = pg_malloc(10);
+	char	   *result = pg_malloc(11);
 
 	if ((sz % 1024) == 0)
-		snprintf(result, 10, "%dGB", sz / 1024);
+		snprintf(result, 11, "%dGB", sz / 1024);
 	else
-		snprintf(result, 10, "%dMB", sz);
+		snprintf(result, 11, "%dMB", sz);
 
 	return result;
 }