refactor: Avoid &foo[0] on C-Style arrays

This is confusing at best when parts of a class use the
redundant operators and other parts do not.

src/key.h

@@ -151,7 +151,7 @@ struct CExtKey {
     friend bool operator==(const CExtKey& a, const CExtKey& b)
     {
         return a.nDepth == b.nDepth &&
-            memcmp(&a.vchFingerprint[0], &b.vchFingerprint[0], sizeof(vchFingerprint)) == 0 &&
+            memcmp(a.vchFingerprint, b.vchFingerprint, sizeof(vchFingerprint)) == 0 &&
             a.nChild == b.nChild &&
             a.chaincode == b.chaincode &&
             a.key == b.key;