Initialize last_sample_ to fix maybe-uninitialized warning.

Change-Id: I794b61e14df751b0b8850f87f5dd8044ab70c1cb
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/83107
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/fipsmodule/entropy/jitter.cc.inc

diff --git a/crypto/fipsmodule/entropy/jitter.cc.inc b/crypto/fipsmodule/entropy/jitter.cc.inc
index af9993b..19c7fbf 100644
--- a/crypto/fipsmodule/entropy/jitter.cc.inc
+++ b/crypto/fipsmodule/entropy/jitter.cc.inc
@@ -153,7 +153,7 @@
  private:
   bool initialized_ = false;
   T sub_sampler_;
-  uint64_t last_sample_;
+  uint64_t last_sample_ = 0;
 };
 
 template <typename U>