awk: fix segfault on for loop syntax error

Parsing "for()" segfaults as awk fails to find loop iteration expressions.

Signed-off-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -316,6 +316,9 @@ testing "awk continue" \
 	"" \
 	'BEGIN { if (1) continue; else a = 1 }'
 
+testing "awk handles invalid for loop" \
+    "awk '{ for() }' 2>&1" "awk: cmd. line:1: Unexpected token\n" "" ""
+
 # testing "description" "command" "result" "infile" "stdin"
 
 exit $FAILCOUNT