Fix logic inversion in soft reference clearing

Add zygote check.

Change-Id: Ibc0dcacbfcfcd0fa66de66ee7fe08d0258d7e2fb
diff --git a/src/mark_sweep.cc b/src/mark_sweep.cc
index b80d1f6..72b9ddb 100644
--- a/src/mark_sweep.cc
+++ b/src/mark_sweep.cc
@@ -581,7 +581,7 @@
 
   // Unless we are in the zygote or required to clear soft references
   // with white references, preserve some white referents.
-  if (clear_soft) {
+  if (!clear_soft && !Runtime::Current()->IsZygote()) {
     PreserveSomeSoftReferences(soft_references);
   }