Add two extra checks to ensure there are no negative array indices.

src/sprs.c

@@ -36,7 +36,7 @@ void sprs_put(sprs* sprs, int index, void* data){
   }
 }
 int sprs_contains(sprs* sprs, int index){
-  return sprs->sparse[index].index < sprs->size && sprs->dense[sprs->sparse[index].index] == index;
+  return index >= 0 && sprs->sparse[index].index >= 0 && sprs->sparse[index].index < sprs->size && sprs->dense[sprs->sparse[index].index] == index;
 }
 void* sprs_get(sprs* sprs, int index){
   void* data = NULL;