MIPS: Fix NaN handling for start index in ArraySplice.

Casting NaN to int is unpredictable, on different architectures it produces different int value.

TEST=test262/S15.4.4.10_A2.1_T2, S15.4.4.10_A2.2_T2, S15.4.4.12_A2.1_T2

BUG=

Review URL: https://codereview.chromium.org/14257006
Patch from Dusan Milosavljevic <Dusan.Milosavljevic@rt-rk.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14426 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/builtins.cc

@@ -934,7 +934,7 @@ BUILTIN(ArraySplice) {
       if (start < kMinInt || start > kMaxInt) {
         return CallJsBuiltin(isolate, "ArraySplice", args);
       }
-      relative_start = static_cast<int>(start);
+      relative_start = std::isnan(start) ? 0 : static_cast<int>(start);
     } else if (!arg1->IsUndefined()) {
       return CallJsBuiltin(isolate, "ArraySplice", args);
     }