Trac #33210: fix uninitialized pointer use in polynomial_modn_dense_n…

…tl.pyx.

Cython is kind enough to point out the following,

  warning: sage/rings/polynomial/polynomial_modn_dense_ntl.pyx:876:32:
  local variable 'mod' referenced before assignment

which references the following (paraphrased) code:

  cdef zz_pX_Modulus_c *mod
  zz_pX_Modulus_build(mod[0], (<Polynomial_dense_modn_ntl_zz>modulus).x)

That's declaring a pointer "mod", and then overwriting the structure
that it points to with pre-computed information about "modulus.x". But
where does "mod" point?

Instead, the first line should be allocating space for a structure
that can then be passed by reference directly (i.e. without indexing)
to zz_pX_Modulus_build. This fix is made in two places, and as a
side-effect, eliminates the Cython warnings.

src/sage/rings/polynomial/polynomial_modn_dense_ntl.pyx

@@ -858,7 +858,7 @@ cdef class Polynomial_dense_modn_ntl_zz(Polynomial_dense_mod_n):
             return self.parent(1)
 
         cdef Polynomial_dense_modn_ntl_zz r = self._new()
-        cdef zz_pX_Modulus_c *mod
+        cdef zz_pX_Modulus_c mod
 
         self.c.restore_c()
 
@@ -873,11 +873,11 @@ cdef class Polynomial_dense_modn_ntl_zz(Polynomial_dense_mod_n):
         else:
             if not isinstance(modulus, Polynomial_dense_modn_ntl_zz):
                 modulus = self.parent()._coerce_(modulus)
-            zz_pX_Modulus_build(mod[0], (<Polynomial_dense_modn_ntl_zz>modulus).x)
+            zz_pX_Modulus_build(mod, (<Polynomial_dense_modn_ntl_zz>modulus).x)
 
             do_sig = zz_pX_deg(self.x) * e * self.c.p_bits > 1e5
             if do_sig: sig_on()
-            zz_pX_PowerMod_long_pre(r.x, self.x, e, mod[0])
+            zz_pX_PowerMod_long_pre(r.x, self.x, e, mod)
             if do_sig: sig_off()
 
         if recip:
@@ -1416,7 +1416,7 @@ cdef class Polynomial_dense_modn_ntl_ZZ(Polynomial_dense_mod_n):
         if self == 0 and e == 0:
             return self.parent(1)
         cdef Polynomial_dense_modn_ntl_ZZ r = self._new()
-        cdef ZZ_pX_Modulus_c *mod
+        cdef ZZ_pX_Modulus_c mod
 
         self.c.restore_c()
 
@@ -1431,11 +1431,11 @@ cdef class Polynomial_dense_modn_ntl_ZZ(Polynomial_dense_mod_n):
         else:
             if not isinstance(modulus, Polynomial_dense_modn_ntl_ZZ):
                 modulus = self.parent()._coerce_(modulus)
-            ZZ_pX_Modulus_build(mod[0], (<Polynomial_dense_modn_ntl_ZZ>modulus).x)
+            ZZ_pX_Modulus_build(mod, (<Polynomial_dense_modn_ntl_ZZ>modulus).x)
 
             do_sig = ZZ_pX_deg(self.x) * e * self.c.p_bits > 1e5
             if do_sig: sig_on()
-            ZZ_pX_PowerMod_long_pre(r.x, self.x, e, mod[0])
+            ZZ_pX_PowerMod_long_pre(r.x, self.x, e, mod)
             if do_sig: sig_off()
         if recip:
             return ~r