apply trac 31645 handling of constant term in series

commit 9aec019
Author: Dave Witte Morris <Dave.Morris@uleth.ca>
Date:   Sun Apr 11 12:09:48 2021 -0600

    trac 31645 handling of constant term in series

src/sage/symbolic/ginac/useries.cpp

@@ -462,7 +462,16 @@ void symbol::useries(flint_series_t& fp, int order) const
 
 void add::useries(flint_series_t& fp, int order) const
 {
-        fmpq_poly_set_ui(fp.ft, 0);
+        const numeric& oc = overall_coeff;
+        if (oc.is_zero())
+                fmpq_poly_set_ui(fp.ft, 0);
+        else if (oc.is_long())
+                fmpq_poly_set_si(fp.ft, oc.to_long());
+        else if (oc.is_mpz())
+                fmpq_poly_set_mpz(fp.ft, oc.as_mpz());
+        else
+                fmpq_poly_set_mpq(fp.ft, oc.as_mpq());
+
         for (const auto & elem : seq) {
 		const ex& t = recombine_pair_to_ex(elem);
                 flint_series_t fp1;
@@ -477,18 +486,6 @@ void add::useries(flint_series_t& fp, int order) const
                 }
                 fmpq_poly_add(fp.ft, fp.ft, fp1.ft);
         }
-        const numeric& oc = overall_coeff;
-        if (oc.is_zero())
-                return;
-
-        flint_series_t fp1;
-        if (oc.is_long())
-                fmpq_poly_set_si(fp1.ft, oc.to_long());
-        else if (oc.is_mpz())
-                fmpq_poly_set_mpz(fp1.ft, oc.as_mpz());
-        else
-                fmpq_poly_set_mpq(fp1.ft, oc.as_mpq());
-        fmpq_poly_add(fp.ft, fp.ft, fp1.ft);
 }
 
 void mul::useries(flint_series_t& fp, int order) const