Fix warnings about unhandled enum value in switch [-Wswitch]

Fixes GCC 5 warnings of the form:

core/io/http_client.cpp:288:9: warning: enumeration value 'STATUS_SSL_HANDSHAKE_ERROR' not handled in switch [-Wswitch]
core/io/marshalls.cpp:806:9: warning: enumeration value 'AABB' not handled in switch [-Wswitch]

Those can be trivial cases where adding a default fallback is the solution,
or more complex issues/hidden bugs where missed values are actually meant
to be handled.

core/os/dir_access.cpp

@@ -227,6 +227,7 @@ String DirAccess::fix_path(String p_path) const {
 
 			return p_path;
 		} break;
+		case ACCESS_MAX: break; // Can't happen, but silences warning
 	}
 
 	return p_path;