libcom_err: Fix sign-extension problem on 64-bit systems in error_message()

libcom_err: Fix sign-extension problem on 64-bit systems in error_message()

On 64-bit systems (or anything with sizeof(long) > sizeof(int)), we
sometimes get error codes passed to error_message which have been cast
from an (int) to an (unsigned int). This almost always happens if
you're using libgssapi_krb5, which returns an error code which is less
than 0 but is returned in an (unsigned int).

For example, -1765328377L gets cast to 2529638919, which is
0x96c73a07, not 0xffffffff96c73a07, so error_message() fails to find a
matching error table.

When error_message() then calls the error_table_name() function to get a
name to use in the "unknown code" message, it gets a correct value back.

This happens because error_table_name() drops most of the higher bits of
the parameter it's passed before doing anything else with it (& 077777777f,
or & 0xffffff). If we did the same thing in error_message(), we wouldn't
have a problem there, either.

Problem reported and fixed by: Nalin Dahyabhai

Addresses-Sourceforge-Bug: #1809658

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>