The 2 arg OpenSSL sk_find() returned -1 on error and >= 0 on
success.  BoringSSL's 3 arg sk_find() returns -1 if the sk argument
is NULL, 0 if the item is not found, and 1 if found.

In practice, all callers of the sk_find() macros in BoringSSL only
check for zero/non-zero.  If sk is ever NULL, it looks like most
callers are going to use uninitialized data as the index because
the return value check is insufficient.

Change-Id: I640089a0f4044aaa8d50178b2aecd9c3c1fe2f9c
Reviewed-on: https://boringssl-review.googlesource.com/4500
Reviewed-by: Adam Langley <agl@google.com>