sed: a communal variable managed to slip past 'size'

('size' happily displays 0 bytes in data and bss,
but in reality sed.o used 180 bytes of it). Oh well.
function                                             old     new   delta
pipe_putc                                             67      76      +9
sed_main                                             627     633      +6
get_next_line                                        161     166      +5
bbg                                                  180       -    -180
(add/remove: 0/1 grow/shrink: 3/0 up/down: 20/-180)          Total: -160 bytes

sed: also make sed -i failure message less cryptic

docs/keep_data_small.txt

@@ -112,7 +112,7 @@ Be careful, though, and use it only if globals fit into bb_common_bufsiz1.
 Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change
 from one libc to another, you have to add compile-time check for it:
 
-if(sizeof(struct globals) > sizeof(bb_common_bufsiz1))
+if (sizeof(struct globals) > sizeof(bb_common_bufsiz1))
 	BUG_<applet>_globals_too_big();