awk: fix use after free (CVE-2022-30065)

fixes https://bugs.busybox.net/show_bug.cgi?id=14781

function                                             old     new   delta
evaluate                                            3343    3357     +14

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -479,4 +479,10 @@ testing 'awk backslash+newline eaten with no trace' \
 	"Hello world\n" \
 	'' ''
 
+testing 'awk assign while test' \
+	"awk '\$1==\$1=\"foo\" {print \$1}'" \
+	"foo\n" \
+	"" \
+	"foo"
+
 exit $FAILCOUNT