Fixed underflow in ev_vec_pop

ev_vec.h

@@ -45,7 +45,8 @@ typedef void *ev_svec_t;
 
 typedef enum {
   EV_VEC_ERR_NONE = 0,
-  EV_VEC_ERR_OOM = 1
+  EV_VEC_ERR_OOM = -1,
+  EV_VEC_ERR_INVALID_OP = -2
 } ev_vec_error_t;
 TYPEDATA_GEN(ev_vec_error_t, DEFAULT(EV_VEC_ERR_NONE));
 
@@ -252,7 +253,7 @@ ev_vec_fini(
  * vector
  *
  * \returns The index of the element that was just pushed. If the operation
- * failed, a non-zero (vec_error_t) value is returned.
+ * failed, a less-than-zero (vec_error_t) value is returned.
  */
 EV_VEC_API int
 ev_vec_push_impl(
@@ -640,6 +641,8 @@ ev_vec_pop(
   ev_vec_t* v = (ev_vec_t*)vec_p;
   __ev_vec_getmeta(*v)
 
+  if(metadata->length == 0) return EV_VEC_ERR_INVALID_OP;
+
   if(out != NULL) {
     void *src = ((char *)*v) + ((metadata->length-1) * metadata->typeData.size);
     if (metadata->typeData.copy_fn) {