awk: allow empty fuinctions with no arguments, disallow function redefinitions

function                                             old     new   delta
.rodata                                           103681  103700     +19
parse_program                                        303     307      +4
evaluate                                            3145    3141      -4
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 23/-4)              Total: 19 bytes

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

testsuite/awk.tests

@@ -44,6 +44,16 @@ testing "awk handles empty function f(arg){}" \
 	"L1\n\nL2\n\n" \
 	"" ""
 
+prg='
+function empty_fun(){}
+END {empty_fun()
+  print "Ok"
+}'
+testing "awk handles empty function f(){}" \
+	"awk '$prg'" \
+	"Ok\n" \
+	"" ""
+
 prg='
 function outer_fun() {
   return 1