1) Replace long-winded abort() construct by assert().

2) Remove micro optimization (inline checking for NaN before calling
   mpd_qcheck_nans()) that probably has no benefit in this case.

Modules/_decimal/libmpdec/mpdecimal.c

@@ -5713,14 +5713,15 @@ void
 mpd_qnext_minus(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
                 uint32_t *status)
 {
-    mpd_context_t workctx; /* function context */
+    mpd_context_t workctx;
     MPD_NEW_CONST(tiny,MPD_POS,mpd_etiny(ctx)-1,1,1,1,1);
 
     if (mpd_isspecial(a)) {
         if (mpd_qcheck_nan(result, a, ctx, status)) {
             return;
         }
-        if (mpd_isinfinite(a)) {
+
+        assert(mpd_isinfinite(a));
         if (mpd_isnegative(a)) {
             mpd_qcopy(result, a, status);
             return;
@@ -5731,13 +5732,10 @@ mpd_qnext_minus(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
             if (mpd_isnan(result)) {
                 return;
             }
-                result->exp = ctx->emax - ctx->prec + 1;
+            result->exp = mpd_etop(ctx);
             return;
         }
     }
-        /* debug */
-        abort(); /* GCOV_NOT_REACHED */
-    }
 
     mpd_workcontext(&workctx, ctx);
     workctx.round = MPD_ROUND_FLOOR;
@@ -5769,7 +5767,8 @@ mpd_qnext_plus(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
         if (mpd_qcheck_nan(result, a, ctx, status)) {
             return;
         }
-        if (mpd_isinfinite(a)) {
+
+        assert(mpd_isinfinite(a));
         if (mpd_ispositive(a)) {
             mpd_qcopy(result, a, status);
         }
@@ -5784,7 +5783,6 @@ mpd_qnext_plus(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
         }
         return;
     }
-    }
 
     mpd_workcontext(&workctx, ctx);
     workctx.round = MPD_ROUND_CEILING;
@@ -5814,8 +5812,7 @@ mpd_qnext_toward(mpd_t *result, const mpd_t *a, const mpd_t *b,
 {
     int c;
 
-    if (mpd_isnan(a) || mpd_isnan(b)) {
-        if (mpd_qcheck_nans(result, a, b, ctx, status))
+    if (mpd_qcheck_nans(result, a, b, ctx, status)) {
         return;
     }