Fix bug in ada_print_floating

ada_print_floating rewrites a floating-point string representation to
conform to Ada syntax.  However, if you managed to get a floating
point error, you might see:

    (gdb) print whatever
    $2 = <invalid float valu.0e>

What's happening here is that ada_print_floating doesn't recognize
this error case, and proceeds to modify the error text.

This patch fixes this problem.