Faster get_unsafe for NTL GF(p) polynomials

Unlike similar Zmod(n) polynomials, the GF(p) NTL polynomials
would call the IntegerModRing constructor for each get_unsafe call
resulting in very slow polynomial evaluation.

Author: remyoudompheng

src/sage/libs/ntl/ntl_ZZ_pX.pyx

@@ -264,9 +264,7 @@ cdef class ntl_ZZ_pX():
         if i < 0:
             r.set_from_int(0)
         else:
-            sig_on()
             r.x = ZZ_pX_coeff( self.x, i)
-            sig_off()
         return r
 
     cdef int getitem_as_int(ntl_ZZ_pX self, long i):

src/sage/rings/polynomial/polynomial_modn_dense_ntl.pyx

@@ -209,7 +209,7 @@ cdef class Polynomial_dense_mod_n(Polynomial):
             sage: f[:3]
             13*x^2 + 10*x + 5
         """
-        return self._parent._base(self.__poly[n]._sage_())
+        return self._parent._base((<ntl_ZZ_pX> self.__poly)[n]._integer_())
 
     def _unsafe_mutate(self, n, value):
         n = int(n)