util: add some defensive coding in u_upload_alloc()

util: add some defensive coding in u_upload_alloc()

Some callers of this function were checking the 'ptr' result to see if
the function failed.  But the correct way is to check the regular
return value for PIPE_ERROR_x.  Now we initialize all the returned
values at the top of the function in case we do hit an error (like OOM).

Callers are more likely to detect OOM conditions now.  But there
are some callers which don't do any error checking...

Note: This is a candidate for the 9.0 branch.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>