box/error: don't allow overflow of error ref counter

There's no overflow check while incrementing error's reference counter
in error_ref(). Meanwhile, stubborn users still may achieve overflow:
each call of box.error.last() increments reference counter of error
residing in diagnostic area. As a result, 2^32 calls of box.error.last()
in a row will lead to counter overflow ergo - to unpredictable results.
Let's fix it and introduce dummy check in error_ref().

src/lib/core/diag.c

@@ -80,6 +80,7 @@ error_set_prev(struct error *e, struct error *prev)
 			 */
 			error_unlink_effect(prev);
 		}
+		assert(prev->refs < INT32_MAX);
 		error_ref(prev);
 		prev->effect = e;
 	}

src/lib/core/diag.h

@@ -112,10 +112,14 @@ struct error {
 	struct error *effect;
 };
 
-static inline void
+static inline int
 error_ref(struct error *e)
 {
+	assert(e->refs >= 0);
+	if (e->refs >= INT32_MAX)
+		return -1;
 	e->refs++;
+	return 0;
 }
 
 void
@@ -230,6 +234,7 @@ static inline void
 diag_set_error(struct diag *diag, struct error *e)
 {
 	assert(e != NULL);
+	assert(e->refs < INT32_MAX);
 	error_ref(e);
 	diag_clear(diag);
 	error_unlink_effect(e);
@@ -254,6 +259,7 @@ diag_add_error(struct diag *diag, struct error *e)
 	 */
 	assert(e->effect == NULL);
 	assert(diag->last->effect == NULL);
+	assert(e->refs < INT32_MAX);
 	error_ref(e);
 	e->cause = diag->last;
 	diag->last->effect = e;

src/lua/error.c

@@ -84,7 +84,8 @@ luaT_pusherror(struct lua_State *L, struct error *e)
 	 * It also important to reference the error first and only
 	 * then set the finalizer.
 	 */
-	error_ref(e);
+	if (error_ref(e) != 0)
+		luaL_error(L, "Too many references to error object");
 	assert(CTID_CONST_STRUCT_ERROR_REF != 0);
 	struct error **ptr = (struct error **)
 		luaL_pushcdata(L, CTID_CONST_STRUCT_ERROR_REF);

src/lua/error.lua

@@ -129,6 +129,10 @@ local function error_set_prev(err, prev)
     if not ffi.istype('struct error', prev) and prev ~= nil then
         error("Usage: error1:set_prev(error2)")
     end
+    local INT32_MAX = 2147483647
+    if err._refs >= INT32_MAX then
+        error("Too many references to error object")
+    end
     local ok = ffi.C.error_set_prev(err, prev);
     if ok ~= 0 then
         error("Cycles are not allowed")