awk: fix FS assignment behavior. Closes 5108

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

testsuite/awk.tests

@@ -217,4 +217,11 @@ end d
 testing "awk handles empty ()" \
 	"awk 'BEGIN {print()}' 2>&1" "awk: cmd. line:1: Empty sequence\n" "" ""
 
+testing "awk FS assignment" "awk '{FS=\":\"; print \$1}'" \
+	"a:b\ne\n" \
+	"" \
+	"a:b c:d\ne:f g:h"
+
+# testing "description" "command" "result" "infile" "stdin"
+
 exit $FAILCOUNT