Fix gcc 4.9 build errors.

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 0ef66535c5301800f038500af7f04f9c2ba2b515..fb8b33844d5bb108525de5c6ed0a05c5a2d40223 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -1417,9 +1417,9 @@ bool RunMainIsolate(const char* script_name,
       uint8_t* assembly_buffer = NULL;
       intptr_t assembly_size = 0;
       uint8_t* instructions_blob_buffer = NULL;
-      intptr_t instructions_blob_size = NULL;
+      intptr_t instructions_blob_size = 0;
       uint8_t* rodata_blob_buffer = NULL;
-      intptr_t rodata_blob_size = NULL;
+      intptr_t rodata_blob_size = 0;
       if (use_blobs) {
         result = Dart_CreatePrecompiledSnapshotBlob(
             &vm_isolate_buffer,