Don't initialize decryptor on CDM creation failure

PpapiDecryptor::Create() must not proceed with decryptor initialization
if the CDM wrapper could not be created.
PpapiDecryptor::PpapiDecryptor() asserts the CDM wrapper is valid and
InitializeCdm() uses it unconditionally.

BUG=488363
TEST=EME tests pass

Review URL: https://codereview.chromium.org/1128693004

Cr-Commit-Position: refs/heads/master@{#330150}
1 file changed