Catch std::underflow_error as well as std::overflow_error.

Any operation that can overflow will throw an underflow if it's a
negative number. The C interface needs to catch both to prevent
unhandled exception crashes,

libgnucash/engine/gnc-numeric.cpp

@@ -391,6 +391,13 @@ GncNumeric::to_decimal(unsigned int max_places) const
             << " overflows when attempting to convert it to decimal.\n";
         throw std::range_error(msg.str());
     }
+    catch (const std::underflow_error& err)
+    {
+        std::ostringstream msg;
+        msg << "GncNumeric " << *this
+            << " underflows when attempting to convert it to decimal.\n";
+        throw std::range_error(msg.str());
+    }
 }
 
 void
@@ -998,6 +1005,10 @@ gnc_numeric_convert(gnc_numeric in, int64_t denom, int how)
     {
         return gnc_numeric_error(GNC_ERROR_OVERFLOW);
     }
+    catch (const std::underflow_error& err)
+    {
+        return gnc_numeric_error(GNC_ERROR_OVERFLOW);
+    }
 }