don't crash when removing out of bounds

ds.c

@@ -67,7 +67,7 @@ void
 		return NULL;
 	}
 
-	for (i = -1, cur = *ll;; i++, cur = cur->next) {
+	for (i = -1, cur = *ll; cur; i++, cur = cur->next) {
 		if (i + 1 == idx) {
 			if (idx == 0) {
 				rm = cur;
@@ -86,6 +86,10 @@ void
 		}
 	}
 
+	if (!rm) {
+		return NULL;
+	}
+
 	data = rm->data;
 	free(rm);
 	return data;
@@ -127,6 +131,10 @@ void
 		}
 	}
 
+	if (!rm) {
+		return NULL;
+	}
+
 	data = rm->data;
 	free(rm);
 	return data;

test.c

@@ -17,6 +17,7 @@ main(int argc, char *argv[])
 	ds_ll_foreach(ds_sll_t, ll) {
 		puts(cur->data);
 	}
+	ds_sll_remove(&ll, 1234);
 	char *pa = ds_sll_remove(&ll, 0); // [ "two" ]
 	char *pb = ds_sll_remove(&ll, 0); // [ ]