Ensure intialization of offset

If we don't initialize offset here its possible that a key selector
could apply the provided offset to an uninitialized value passed in from
the calling function.
diff --git a/c_src/util.c b/c_src/util.c
index 0ede4a2..852c083 100644
--- a/c_src/util.c
+++ b/c_src/util.c
@@ -66,6 +66,8 @@
         return 0;
     }
 
+    *offset = 0;
+
     if(arity >= 2) {
         if(IS_ATOM(tuple[1], lt)) {
             *or_equal = 0;