fix strerror_r test

e6e60065ff093ff8c859ab146cf543531cb1967c modified strerror_r to
treat errno as signed. However, the change to the test code
modified the "strerror" test, not the "strerror_r" test.

Make the same change for the strerror_r code.

Change-Id: Ia236a53df5745935e229a4446a74da8bed0cfd7b