Fix bug that would cause a double free if setup files and free is called

src/sprs.c

@@ -14,6 +14,8 @@ libds_result sprs_setup(sprs* sprs, int size){
   if(sprs->sparse == NULL || sprs->dense == NULL){
     free(sprs->sparse);
     free(sprs->dense);
+    sprs->sparse = NULL;
+    sprs->dense = NULL;
     result = LIBDS_MALLOC;
   } else {
     sprs->capacity = size;