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.cc b/runtime/common_runtime_test.cc
index 78ba6e7..15724a1 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -68,6 +68,74 @@
 
 using android::base::StringPrintf;
 
+static const uint8_t kBase64Map[256] = {
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255,  62, 255, 255, 255,  63,
+  52,  53,  54,  55,  56,  57,  58,  59,  60,  61, 255, 255,
+  255, 254, 255, 255, 255,   0,   1,   2,   3,   4,   5,   6,
+    7,   8,   9,  10,  11,  12,  13,  14,  15,  16,  17,  18,  // NOLINT
+   19,  20,  21,  22,  23,  24,  25, 255, 255, 255, 255, 255,  // NOLINT
+  255,  26,  27,  28,  29,  30,  31,  32,  33,  34,  35,  36,
+   37,  38,  39,  40,  41,  42,  43,  44,  45,  46,  47,  48,  // NOLINT
+   49,  50,  51, 255, 255, 255, 255, 255, 255, 255, 255, 255,  // NOLINT
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+  255, 255, 255, 255
+};
+
+uint8_t* DecodeBase64(const char* src, size_t* dst_size) {
+  CHECK(dst_size != nullptr);
+  std::vector<uint8_t> tmp;
+  uint32_t t = 0, y = 0;
+  int g = 3;
+  for (size_t i = 0; src[i] != '\0'; ++i) {
+    uint8_t c = kBase64Map[src[i] & 0xFF];
+    if (c == 255) continue;
+    // the final = symbols are read and used to trim the remaining bytes
+    if (c == 254) {
+      c = 0;
+      // prevent g < 0 which would potentially allow an overflow later
+      if (--g < 0) {
+        *dst_size = 0;
+        return nullptr;
+      }
+    } else if (g != 3) {
+      // we only allow = to be at the end
+      *dst_size = 0;
+      return nullptr;
+    }
+    t = (t << 6) | c;
+    if (++y == 4) {
+      tmp.push_back((t >> 16) & 255);
+      if (g > 1) {
+        tmp.push_back((t >> 8) & 255);
+      }
+      if (g > 2) {
+        tmp.push_back(t & 255);
+      }
+      y = t = 0;
+    }
+  }
+  if (y != 0) {
+    *dst_size = 0;
+    return nullptr;
+  }
+  std::unique_ptr<uint8_t[]> dst(new uint8_t[tmp.size()]);
+  *dst_size = tmp.size();
+  std::copy(tmp.begin(), tmp.end(), dst.get());
+  return dst.release();
+}
+
 ScratchFile::ScratchFile() {
   // ANDROID_DATA needs to be set
   CHECK_NE(static_cast<char*>(nullptr), getenv("ANDROID_DATA")) <<