Fix a bug when LEDA is used instead of GMP

That is a really trivial bug fix, for a rather nasty bug! I spend
several hours on it! Currently the `leda::rational::operator>>` does not
clear the `failbit` of the stream, when it reads a number without
denominator, and the EOF directly after. The flag `eofbit` and `failbit`
are set while trying to read the '/', but in that case the `failbit`
should be cleared from the stream status.

Trivial bug fix for master.

Boolean_set_operations_2/test/Boolean_set_operations_2/data/pgn_holes4.dat

@@ -34,4 +34,4 @@
 3
 7 4
 7 5
-8 4
+8 4

Boolean_set_operations_2/test/Boolean_set_operations_2/data/pgn_holes5.dat

@@ -17,4 +17,4 @@
 3
 4 0
 4 2
-6 3
+6 3