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/bitcoin-cli.cpp

@@ -366,7 +366,7 @@ static UniValue CallRPC(BaseRequestHandler *rh, const std::string& strMethod, co
     std::string endpoint = "/";
     if (!gArgs.GetArgs("-rpcwallet").empty()) {
         std::string walletName = gArgs.GetArg("-rpcwallet", "");
-        char *encodedURI = evhttp_uriencode(walletName.c_str(), walletName.size(), false);
+        char *encodedURI = evhttp_uriencode(walletName.data(), walletName.size(), false);
         if (encodedURI) {
             endpoint = "/wallet/"+ std::string(encodedURI);
             free(encodedURI);

src/crypto/hkdf_sha256_32.cpp

@@ -9,7 +9,7 @@
 
 CHKDF_HMAC_SHA256_L32::CHKDF_HMAC_SHA256_L32(const unsigned char* ikm, size_t ikmlen, const std::string& salt)
 {
-    CHMAC_SHA256((const unsigned char*)salt.c_str(), salt.size()).Write(ikm, ikmlen).Finalize(m_prk);
+    CHMAC_SHA256((const unsigned char*)salt.data(), salt.size()).Write(ikm, ikmlen).Finalize(m_prk);
 }
 
 void CHKDF_HMAC_SHA256_L32::Expand32(const std::string& info, unsigned char hash[OUTPUT_SIZE])

src/fs.cpp

@@ -107,10 +107,10 @@ std::string get_filesystem_error_message(const fs::filesystem_error& e)
 #else
     // Convert from Multi Byte to utf-16
     std::string mb_string(e.what());
-    int size = MultiByteToWideChar(CP_ACP, 0, mb_string.c_str(), mb_string.size(), nullptr, 0);
+    int size = MultiByteToWideChar(CP_ACP, 0, mb_string.data(), mb_string.size(), nullptr, 0);
 
     std::wstring utf16_string(size, L'\0');
-    MultiByteToWideChar(CP_ACP, 0, mb_string.c_str(), mb_string.size(), &*utf16_string.begin(), size);
+    MultiByteToWideChar(CP_ACP, 0, mb_string.data(), mb_string.size(), &*utf16_string.begin(), size);
     // Convert from utf-16 to utf-8
     return std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t>().to_bytes(utf16_string);
 #endif

src/httprpc.cpp

@@ -112,7 +112,7 @@ static bool multiUserAuthorized(std::string strUserPass)
         static const unsigned int KEY_SIZE = 32;
         unsigned char out[KEY_SIZE];
 
-        CHMAC_SHA256(reinterpret_cast<const unsigned char*>(strSalt.c_str()), strSalt.size()).Write(reinterpret_cast<const unsigned char*>(strPass.c_str()), strPass.size()).Finalize(out);
+        CHMAC_SHA256(reinterpret_cast<const unsigned char*>(strSalt.data()), strSalt.size()).Write(reinterpret_cast<const unsigned char*>(strPass.data()), strPass.size()).Finalize(out);
         std::vector<unsigned char> hexvec(out, out+KEY_SIZE);
         std::string strHashFromPass = HexStr(hexvec);
 

src/util/strencodings.cpp

@@ -138,7 +138,7 @@ std::string EncodeBase64(const unsigned char* pch, size_t len)
 
 std::string EncodeBase64(const std::string& str)
 {
-    return EncodeBase64((const unsigned char*)str.c_str(), str.size());
+    return EncodeBase64((const unsigned char*)str.data(), str.size());
 }
 
 std::vector<unsigned char> DecodeBase64(const char* p, bool* pf_invalid)
@@ -207,7 +207,7 @@ std::string EncodeBase32(const unsigned char* pch, size_t len)
 
 std::string EncodeBase32(const std::string& str)
 {
-    return EncodeBase32((const unsigned char*)str.c_str(), str.size());
+    return EncodeBase32((const unsigned char*)str.data(), str.size());
 }
 
 std::vector<unsigned char> DecodeBase32(const char* p, bool* pf_invalid)

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);