keystone2: Fix check of i2d_re_X509_tbs call

We store the result of our call to i2d_re_X509_tbs() in a signed
variable, so our check for negative error values will work.

Test: TreeHugger
Change-Id: I8824c1c703d9c1d238004f9e5ee7f46a9ae2ddce
1 file changed