slsqp: check ptot>n, not p>n (proper fix for stevengj#592)

src/algs/slsqp/slsqp.c (src/algs/slsqp/slsqp.c): Fix the check for more
equality constraints than variables (introduced by PR stevengj#593 to fix stevengj#592)
so that vector constraints (mconstraints) are correctly counted: the
count to consider here is ptot (the total number of scalar equality
constraints), not p (the number of equality constraints that may be
vector constraints).

src/algs/slsqp/slsqp.c

@@ -2459,7 +2459,7 @@ nlopt_result nlopt_slsqp(unsigned n, nlopt_func f, void *f_data,
      unsigned max_cdim;
      int want_grad = 1;
 
-     if (p > n) {
+     if (ptot > n) {
        nlopt_stop_msg(stop, "slsqp: more equality constraints than variables");
        return NLOPT_INVALID_ARGS;
      }