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;

ds.h

@@ -1,14 +1,14 @@
-#ifndef _DS_LOADED
-#define _DS_LOADED
+#ifndef DS_H
+#define DS_H
 
-typedef struct _sll {
-  struct _sll *next;
+typedef struct _ds_sll {
+  struct _ds_sll *next;
   void        *data;
 } ds_sll_t;
 
-typedef struct _dll {
-  struct _dll *next;
-  struct _dll *prev;
+typedef struct _ds_dll {
+  struct _ds_dll *next;
+  struct _ds_dll *prev;
   void        *data;
 } ds_dll_t;
 

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); // [ ]