config.hpp: Increase GRAPHENE_MIN_ACCOUNT_NAME_LENGTH to 3

Length requirement is enforced by RFC 1035 grammar.  Use a compiler
error to tell anyone changing config.hpp that any value for this
constant smaller than 3 is unsupported by the implementation.

libraries/chain/include/graphene/chain/config.hpp

@@ -20,7 +20,7 @@
 #define GRAPHENE_SYMBOL "CORE"
 #define GRAPHENE_ADDRESS_PREFIX "GPH"
 
-#define GRAPHENE_MIN_ACCOUNT_NAME_LENGTH 1
+#define GRAPHENE_MIN_ACCOUNT_NAME_LENGTH 3
 #define GRAPHENE_MAX_ACCOUNT_NAME_LENGTH 63
 
 #define GRAPHENE_MIN_ASSET_SYMBOL_LENGTH 3

libraries/chain/protocol/account.cpp

@@ -34,6 +34,10 @@ namespace graphene { namespace chain {
  */
 bool is_valid_name( const string& name )
 {
+#if GRAPHENE_MIN_ACCOUNT_NAME_LENGTH < 3
+#error This is_valid_name implementation implicitly enforces minimum name length of 3.
+#endif
+
     const size_t len = name.size();
     if( len < GRAPHENE_MIN_ACCOUNT_NAME_LENGTH )
         return false;