Fix use after free bug in PprofCodeObserver.

runtime/vm/os_linux.cc

Index: runtime/vm/os_linux.cc
diff --git a/runtime/vm/os_linux.cc b/runtime/vm/os_linux.cc
index 3c7495b46b0cc7ced5168cce75c97fd99d1ffe3f..400d35366bab3da93886e2db1d809f26cd2963b0 100644
--- a/runtime/vm/os_linux.cc
+++ b/runtime/vm/os_linux.cc
@@ -112,11 +112,11 @@ class PprofCodeObserver : public CodeObserver {
     pprof_symbol_generator_->WriteToMemory(debug_region);
     int buffer_size = debug_region->size();
     void* buffer = debug_region->data();
-    delete debug_region;
     if (buffer_size > 0) {
       ASSERT(buffer != NULL);
       (*file_write)(buffer, buffer_size, out_file);
     }
+    delete debug_region;
     (*file_close)(out_file);
     DebugInfo::UnregisterAllSections();
   }