Fix more missing error checks in v3_cpols.c

I only imported half of upstream's c5f2b5336ab72e40ab91e2ca85639f51fa3178c6 on
accident.

Change-Id: Ice8185ca6770f915eb859e918f5db7d5ccdc7cc7
Reviewed-on: https://boringssl-review.googlesource.com/4045
Reviewed-by: Adam Langley <agl@google.com>