Prevent flint clear of null pointer

Author: Jake-Moss

src/flint/types/fmpq_vec.pyx

@@ -31,7 +31,9 @@ cdef class fmpq_vec:
 
     def __dealloc__(self):
         libc.stdlib.free(self.double_indirect)
-        _fmpq_vec_clear(self.val, self.length)
+        if self.val is not NULL:
+            _fmpq_vec_clear(self.val, self.length)
+            self.val = NULL
 
     def __getitem__(self, x):
         if not isinstance(x, int):

src/flint/types/fmpz_vec.pyx

@@ -32,7 +32,9 @@ cdef class fmpz_vec:
 
     def __dealloc__(self):
         libc.stdlib.free(self.double_indirect)
-        _fmpz_vec_clear(self.val, self.length)
+        if self.val is not NULL:
+            _fmpz_vec_clear(self.val, self.length)
+            self.val = NULL
 
     def __getitem__(self, x):
         if not isinstance(x, int):