fix review comments

Author: keltecc

src/sage/rings/polynomial/multi_polynomial_libsingular.pyx

@@ -2993,7 +2993,7 @@ cdef class MPolynomial_libsingular(MPolynomial_libsingular_base):
             sage: f.monomial_coefficient(x - x)
             Traceback (most recent call last):
             ...
-            ValueError: mon must not be equal to 0.
+            ValueError: mon must not be equal to 0
         """
         cdef poly *p = self._poly
         cdef poly *m = mon._poly
@@ -3002,8 +3002,8 @@ cdef class MPolynomial_libsingular(MPolynomial_libsingular_base):
         if mon._parent is not self._parent:
             raise TypeError("mon must have same parent as self.")
 
-        if mon._poly == NULL:
-            raise ValueError("mon must not be equal to 0.")
+        if mon._poly is NULL:
+            raise ValueError("mon must not be equal to 0")
 
         while p:
             if p_ExpVectorEqual(p, m, r) == 1: