Free up ADB and CHOICE if already initialised.

CVE-2015-0287

(Imported from upstream's b485d976340d3ca080060c3c7dee9102e2200762.)

Change-Id: Ie8dac46ccc95a4eee9ef535d8620217202f13ca1
Reviewed-on: https://boringssl-review.googlesource.com/4046
Reviewed-by: Adam Langley <agl@google.com>