[roll] Roll fuchsia [audio_core] Make safe-write pos more conservative

This CL changes our safe-write frame calculation (in
WritableRingBuffer) to Ceiling instead of Floor, to be
optimally conservative ("safe").

Test: manual testing with Sherlock

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/665165
Original-Revision: 91bd1d0b04421db57a11a7a401051fd0cace107e
GitOrigin-RevId: eb33745b4170514d8b5a068d9ce79c00599342e1
Change-Id: Iedc8257ce2d0e87c491769317e485687faff6737
diff --git a/stem b/stem
index 2132426..c6379e3 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="bb323114145d8f04ffd1049cac74233342fb07e5"/>
+             revision="91bd1d0b04421db57a11a7a401051fd0cace107e"/>
   </projects>
   <hooks>
     <hook name="install-environment"