Fix occurences of c_str() used with size() to data()

Using `data()` better communicates the intent here.

Also, depending on how `c_str()` is implemented, this fixes undefined
behavior: The part of the string after the first NULL character might
have undefined contents.

src/wallet/crypter.cpp

@@ -23,7 +23,7 @@ int CCrypter::BytesToKeySHA512AES(const std::vector<unsigned char>& chSalt, cons
     unsigned char buf[CSHA512::OUTPUT_SIZE];
     CSHA512 di;
 
-    di.Write((const unsigned char*)strKeyData.c_str(), strKeyData.size());
+    di.Write((const unsigned char*)strKeyData.data(), strKeyData.size());
     di.Write(chSalt.data(), chSalt.size());
     di.Finalize(buf);