Restrict a2b_hex to 2000 bytes. am: 9a659e31c1

Original change: https://googleplex-android-review.googlesource.com/c/platform/vendor/widevine/+/13805068

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ibdaaf80a067aff3764062ec1f698b68ff9e8e6fa

libwvdrmengine/cdm/util/src/string_conversions.cpp

@@ -254,6 +254,8 @@ std::vector<uint8_t> Base64SafeDecode(const std::string& b64_input) {
 std::string HexEncode(const uint8_t* in_buffer, unsigned int size) {
   static const char kHexChars[] = "0123456789ABCDEF";
   if (size == 0) return "";
+  constexpr unsigned int kMaxSafeSize = 2048;
+  if (size > kMaxSafeSize) size = kMaxSafeSize;
   // Each input byte creates two output hex characters.
   std::string out_buffer(size * 2, '\0');