off-by-one max exponent computation in convert_doublest_to_floatformat

off-by-one max exponent computation in convert_doublest_to_floatformat

Assuming the following variable definition:

    long double inp = 2.0;

On platforms where "long double" is a double precision IEEE flaoting
point, GDB currently behaves as follow:

    (gdb) set variable inp = 1.6e+308l
    (gdb) p inp
    $2 = inf   <<<<---- !!!!

Instead, the value of "inp" should be printed as:

    (gdb) p inp
    $1 = 1.6e+308

The problem is due to a small error in the comparison of the exponent
versus the maximum value this exponent can take, causing us to think
that the value was too big to fit. But it isn't.

gdb/ChangeLog:

        * doublest.c (convert_doublest_to_floatformat): Fix comparison
        against maximum exponent value.

gdb/testsuite/ChangeLog:

        * gdb.base/ldbl_e308.c, gdb.base/ldbl_e308.exp: New files.