Bug 798616 - Can't register amount greater than 9,000,000,000

Because when loading the value the split in the xml backend doesn't yet
have a parent so the code tried to convert to GNC_COMMODITY_MAX_FRACTION
and if the numerator was larger than 10^10 that would overflow.

To fix it this changes the "don't know" response to get_currency_denom
and get_commodity_denom to GNC_DENOM_AUTO which will normally leave
the denominator alone.

libgnucash/engine/Split.c

@@ -1130,13 +1130,9 @@ xaccSplitDetermineGainStatus (Split *split)
 static inline int
 get_currency_denom(const Split * s)
 {
-    if (!s)
+    if (!(s && s->parent && s->parent->common_currency))
     {
-        return 0;
-    }
-    else if (!s->parent || !s->parent->common_currency)
-    {
-        return GNC_COMMODITY_MAX_FRACTION;
+        return GNC_DENOM_AUTO;
     }
     else
     {
@@ -1147,13 +1143,9 @@ get_currency_denom(const Split * s)
 static inline int
 get_commodity_denom(const Split * s)
 {
-    if (!s)
-    {
-        return 0;
-    }
-    else if (!s->acc)
+    if (!(s && s->acc))
     {
-        return GNC_COMMODITY_MAX_FRACTION;
+        return GNC_DENOM_AUTO;
     }
     else
     {