Merge #17280: refactor: Change occurences of c_str() used with size() to data()

f3b51eb935 Fix occurences of c_str() used with size() to data() (Wladimir J. van der Laan)

Pull request description:

  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 (or even be inaccessible, worst case).~~ Apparently [this is no longer an issue with C++11](https://github.com/bitcoin/bitcoin/pull/17281#discussion_r339742128).

ACKs for top commit:
  fjahr:
    Code review ACK f3b51eb
  practicalswift:
    ACK f3b51eb935 -- diff looks correct, `data()` more idiomatic
  ryanofsky:
    Code review ACK f3b51eb935. Most of these calls (including one in crypter.cpp) are passing text strings, not binary strings likely to contain `\0` and were probably safe before, but much better to avoid the possibility of bugs like this.

Tree-SHA512: 842e1bdd37efc4ece2ecb87ca34962aafef0a192180051def630607e349dc9c8b4e562481fff3de474515f493b4ee3ea53b00269a801a66e625326a38dfce5b8

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)