conf has the ability to expand variables in config files. Repeatedly doing
this can lead to an exponential increase in the amount of memory required.
This places a limit on the length of a value that can result from an
expansion.

Credit to OSS-Fuzz for finding this problem.

(Imported from upstream's 6a6213556a. This
also import's upstream's ee1ccd0a41 which
we had previously missed.)

Change-Id: I9be06a7e8a062b5adcd00c974a7b245226123563
Reviewed-on: https://boringssl-review.googlesource.com/14316
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>