[bugfix] chinrem_gcd: handle polynomials over rationals properly.
* extract_integer_content():
integer_content() can also return a rational number, e.g if the expression
is a polynomial over rationals (in this case expr/content is polynomial
over integers with integer content being 1). Therefore check if
integer_content() is really an integer (and if it's not just return 1,
GCD for polynomials over a field is defined up to arbitrary element of
the field).
This fixes possible segfault when computing GCD of polynomials over
rationals (this is not theoretical, see the added test case).
Thanks to Ernst Moritz Hahn for reporting this bug.
(cherry picked from commit
3d09388a8ea144a19949c216fae43ccba92e47aa)