Fix error handling in bn_exp

In the event of an error |rr| could be NULL. Therefore don't assume you can
use |rr| in the error handling code.

(Imported from upstream's 8c5a7b33c6269c3bd6bc0df6b4c22e4fba03b485.)

Change-Id: I0b392991ce8170dc418e93003af256d535d1e2e8
Reviewed-on: https://boringssl-review.googlesource.com/4005
Reviewed-by: Adam Langley <agl@google.com>
1 file changed