Fix DebugMode error due to overflow in comparison

theano/scalar/basic.py

@@ -199,7 +199,11 @@ class Scalar(Type):
                 type(data), data, self.dtype), e)
 
     def values_eq_approx(self, a, b, tolerance=1e-4):
-        return abs(a - b) <= ((abs(a) + abs(b)) * tolerance)
+        # The addition have risk of overflow especially with [u]int8
+        diff = a - b
+        if diff == 0:
+            return True
+        return abs(diff) <= ((abs(a) + abs(b)) * tolerance)
 
     def c_headers(self, c_compiler):
         l = ['<math.h>']