Fix bug caused by garbage collection

AInvInt can cause a garbage collection, which moves the memory that
ptKnown points to.

src/permutat.c

@@ -1135,6 +1135,8 @@ Obj PowPerm2Int (
 
  /* compute the power by raising the cycles individually for large exps */
  else if ( TNUM_OBJ(opR) == T_INTNEG ) {
+ /* do negation first as it can cause a garbage collection */
+ opR = AInvInt(opR);
 
  /* make sure that the buffer bag is large enough */
  UseTmpPerm(SIZE_OBJ(opL));
@@ -1145,7 +1147,6 @@ Obj PowPerm2Int (
  ptKnown[p] = 0;
 
  /* get pointer to the permutation and the power */
- opR = AInvInt(opR);
  ptL = CONST_ADDR_PERM2(opL);
  ptP = ADDR_PERM2(pow);
 
@@ -1397,6 +1398,8 @@ Obj PowPerm4Int (
 
  /* compute the power by raising the cycles individually for large exps */
  else if ( TNUM_OBJ(opR) == T_INTNEG ) {
+ /* do negation first as it can cause a garbage collection */
+ opR = AInvInt(opR);
 
  /* make sure that the buffer bag is large enough */
  UseTmpPerm(SIZE_OBJ(opL));
@@ -1407,7 +1410,6 @@ Obj PowPerm4Int (
  ptKnown[p] = 0;
 
  /* get pointer to the permutation and the power */
- opR = AInvInt(opR);
  ptL = CONST_ADDR_PERM4(opL);
  ptP = ADDR_PERM4(pow);