awk: fix beavior of "exit" without parameter

function                                             old     new   delta
evaluate                                            3336    3339      +3
awk_exit                                              93      94      +1
awk_main                                             829     827      -2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-2)                Total: 2 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -445,4 +445,9 @@ testing 'awk $NF is empty' \
 	'' \
 	'a=====123='
 
+testing "awk exit N propagates through END's exit" \
+	"awk 'BEGIN { exit 42 } END { exit }'; echo \$?" \
+	"42\n" \
+	'' ''
+
 exit $FAILCOUNT