Segmentation fault in monomial_coefficient

[keltecc]

Steps To Reproduce

P.<x, y> = PolynomialRing(ZZ)
(x + y).monomial_coefficient(x - x)

Expected Behavior

Raise ValueError with error text.

Actual Behavior

Segmentation fault.

Additional Information

There is a missing check for zero monomial in monomial_coefficient() for multivariate polynomial:

sage/src/sage/rings/polynomial/multi_polynomial_libsingular.pyx

Lines 2947 to 3001 in c9dd1e8

	def monomial_coefficient(self, MPolynomial_libsingular mon):
	"""
	Return the coefficient in the base ring of the monomial mon in
	``self``, where mon must have the same parent as ``self``.
	This function contrasts with the function ``coefficient``
	which returns the coefficient of a monomial viewing this
	polynomial in a polynomial ring over a base ring having fewer
	variables.
	INPUT:
	- ``mon`` -- a monomial
	OUTPUT: coefficient in base ring
	.. SEEALSO::
	For coefficients in a base ring of fewer variables,
	look at ``coefficient``.
	EXAMPLES::
	sage: P.<x,y> = QQ[]
	The parent of the return is a member of the base ring.
	sage: f = 2 * x * y
	sage: c = f.monomial_coefficient(x*y); c
	2
	sage: c.parent()
	Rational Field
	sage: f = y^2 + y^2*x - x^9 - 7*x + 5*x*y
	sage: f.monomial_coefficient(y^2)
	1
	sage: f.monomial_coefficient(x*y)
	5
	sage: f.monomial_coefficient(x^9)
	-1
	sage: f.monomial_coefficient(x^10)
	0
	"""
	cdef poly *p = self._poly
	cdef poly *m = mon._poly
	cdef ring *r = self._parent_ring
	if mon._parent is not self._parent:
	raise TypeError("mon must have same parent as self.")
	while p:
	if p_ExpVectorEqual(p, m, r) == 1:
	return si2sa(p_GetCoeff(p, r), r, self._parent._base)
	p = pNext(p)
	return self._parent._base._zero_element

Value mon._poly == NULL is passed to Singular and then NULL dereference occurs:

https://github.com/Singular/Singular/blob/9cdbd118fe4118123a2dd5df729ca3a516305b64/libpolys/polys/monomials/p_polys.h#L1505-L1521

Environment

• OS: Ubuntu 22.04, macOS Ventura 13.0.1
• Sage Version: SageMath version 10.6.beta2

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide