Caught by clang scan-build.

Change-Id: I4f10c879dc137d4a14a7a395764d28e5caa033ff
Reviewed-on: https://boringssl-review.googlesource.com/1342
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)