Bug in demangler

[Tom Tromey]

I'm using the latest gcc 3.0 branch on x86 Red Hat Linux 6.2.

This crashes the demangler:

    ./c++filt _Utf58_0_1__1_2147483647__2147483648

This symbol can be generated by gcj in the right circumstances.

This bug makes it difficult to debug some Java programs, as gdb simply
crashes on startup.

I'm hoping we can change gcj so that it doesn't generate this, but
meanwhile this also represents a real bug in the demangler, which
should never crash.

Tom