Test $GENERATE in nibble mode with a negative value

Negative values used to cause $GENERATE to loop forever.

(cherry picked from commit 32535de856)

bin/tests/system/checkzone/zones/good-generate-modifier.db

@@ -18,3 +18,4 @@ $GENERATE 0-7   host$	A 1.2.3.${1,0,d}
 $GENERATE 8-9   host$	A 1.2.3.${1,0}
 $GENERATE 10-11 host$	A 1.2.3.${1}
 $GENERATE 1024-1026 ${0,3,n}	AAAA 2001:db8::${0,4,x}
+$GENERATE 1024-1026 ${-2000,0,n}	AAAA 2001:db8::${0,4,x}