Skip card mark when storing known null reference

We don't need to mark the gc card if storing a null reference.

Change-Id: I445db196a097ed92b67b8bf1e6b486c6fac7d08f
diff --git a/src/compiler/codegen/mips/int_mips.cc b/src/compiler/codegen/mips/int_mips.cc
index 675cf8d..41d289e 100644
--- a/src/compiler/codegen/mips/int_mips.cc
+++ b/src/compiler/codegen/mips/int_mips.cc
@@ -639,7 +639,9 @@
   StoreBaseIndexed(cu, r_ptr, r_index, r_value, scale, kWord);
   FreeTemp(cu, r_ptr);
   FreeTemp(cu, r_index);
-  MarkGCCard(cu, r_value, r_array);
+  if (!IsConstantNullRef(cu, rl_src)) {
+    MarkGCCard(cu, r_value, r_array);
+  }
 }
 
 bool MipsCodegen::GenShiftImmOpLong(CompilationUnit* cu, Instruction::Code opcode, RegLocation rl_dest,