[roll] Roll fuchsia [starnix] Don't overwrite the old mask when handling a signal

Don't overwrite the old mask when handling a signal.
Previously, if we got one signal while handling another,
we would use the wrong mask.

Original-Fixed: 406803667
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1275327
Original-Revision: 020e3573f403a777ead8ec5251362868ab093546
GitOrigin-RevId: 37c48bfa0cc23da2d0100e15c2db8775353091b3
Change-Id: I4407013a7f42b1cb54a3a37bb949e6ad4435da90
diff --git a/stem b/stem
index e5d5089..fc17fbf 100644
--- a/stem
+++ b/stem
@@ -8,7 +8,7 @@
              root=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="a555b803bec8ae6e2faddc4e935700bebf15c4e4"
+             revision="020e3573f403a777ead8ec5251362868ab093546"
              />
   </imports>
   <projects>