Make base64 explicitly check dst_size for nullptr

Also moved it out of utils.h and into common_runtime_test.h since it
is only used for tests.

Test: ./test.py --host -j40
Change-Id: Ib3c18cb510be833928482c0b2f47f9fb02933f7f
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index d7abe2a..bfa273d 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -44,6 +44,8 @@
 class Runtime;
 typedef std::vector<std::pair<std::string, const void*>> RuntimeOptions;
 
+uint8_t* DecodeBase64(const char* src, size_t* dst_size);
+
 class ScratchFile {
  public:
   ScratchFile();