Improve Underflow handling in the correct-rounding loop. The case for

Underflow to zero hasn't changed: _mpd_qexp() internally uses MIN_EMIN,
so the result would also underflow to zero for all emin > MIN_EMIN.

In case digits are left, the informal argument is as follows: Underflow can
occur only once in the last multiplication of the power stage (in the Horner
stage Underflow provably cannot occur, and if Underflow occurred twice in
the power stage, the result would underflow to zero on the second occasion).

Since there is no double rounding during Underflow, the effective work
precision is now 1 <= result->digits < prec. It can be shown by a somewhat
tedious argument that abs(result - e**x) < ulp(result, result->digits).

Therefore the correct rounding loop now uses ulp(result, result->digits)
to generate the bounds for e**x in case of Underflow.

Modules/_decimal/libmpdec/mpdecimal.c

@@ -4122,6 +4122,8 @@ mpd_qexp(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
         MPD_NEW_STATIC(ulp, 0,0,0,0);
         MPD_NEW_STATIC(aa, 0,0,0,0);
         mpd_ssize_t prec;
+        mpd_ssize_t ulpexp;
+        uint32_t workstatus;
 
         if (result == a) {
             if (!mpd_qcopy(&aa, a, status)) {
@@ -4135,12 +4137,20 @@ mpd_qexp(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
         prec = ctx->prec + 3;
         while (1) {
             workctx.prec = prec;
-            _mpd_qexp(result, a, &workctx, status);
-            _ssettriple(&ulp, MPD_POS, 1,
-                        result->exp + result->digits-workctx.prec);
+            workstatus = 0;
+
+            _mpd_qexp(result, a, &workctx, &workstatus);
+            *status |= workstatus;
+
+            ulpexp = result->exp + result->digits - workctx.prec;
+            if (workstatus & MPD_Underflow) {
+                /* The effective work precision is result->digits. */
+                ulpexp = result->exp;
+            }
+            _ssettriple(&ulp, MPD_POS, 1, ulpexp);
 
             /*
-             * At this point:
+             * At this point [1]:
              *   1) abs(result - e**x) < 0.5 * 10**(-prec) * e**x
              *   2) result - ulp < e**x < result + ulp
              *   3) result - ulp < result < result + ulp
@@ -4148,6 +4158,9 @@ mpd_qexp(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
              * If round(result-ulp)==round(result+ulp), then
              * round(result)==round(e**x). Therefore the result
              * is correctly rounded.
+             *
+             * [1] If abs(a) <= 9 * 10**(-prec-1), use the absolute
+             *     error for a similar argument.
              */
             workctx.prec = ctx->prec;
             mpd_qadd(&t1, result, &ulp, &workctx, &workctx.status);