Resize signature buffer

There is an ambiguity in the use of the signature length in
OEMCrypto_GenerateRSASignature.  If the pointer to the vector is null,
the function should set the length parameter to be the size of the
buffer needed.

This ambiguity has been clarified in the documentation: the
length returned should be the exact length of the buffer.  It
will be corrected in a future release of Qualcomm's L1.  However,
we are putting this change in both as a stop-gap and as a
belt-and-suspenders fix.

Bug: 8878371

Merge of https://widevine-internal-review.googlesource.com/#/c/5494/
from the Widevine CDM repository

Change-Id: I7574874884ca10da68d15674c971a565d015767d

libwvdrmengine/cdm/core/src/crypto_session.cpp

@@ -410,6 +410,9 @@ bool CryptoSession::GenerateSignature(const std::string& message,
     return false;
   }
 
+  // TODO(fredgc): remove in K, when L1 library reports correct length.
+  signature->resize(length);
+
   return true;
 }
 
@@ -556,6 +559,8 @@ bool CryptoSession::RewrapDeviceRSAKey(const std::string& message,
       reinterpret_cast<uint8_t*>(const_cast<char*>(wrapped_rsa_key->data())),
       &wrapped_rsa_key_length);
 
+  // TODO(fredgc): remove in K, when L1 library reports correct length.
+  wrapped_rsa_key->resize(wrapped_rsa_key_length);
   if (OEMCrypto_SUCCESS != status) {
     LOGE("OEMCrypto_RewrapDeviceRSAKey fails with %d", status);
     return false;