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.

modules/gdscript/gdscript_editor.cpp

@@ -1110,6 +1110,7 @@ static bool _guess_expression_type(const GDScriptCompletionContext &p_context, c
 				} break;
 			}
 		} break;
+		default: {}
 	}
 
 	// It may have found a null, but that's never useful
@@ -3354,6 +3355,7 @@ Error GDScriptLanguage::lookup_code(const String &p_code, const String &p_symbol
 				return OK;
 			}
 		} break;
+		default: {}
 	}
 
 	return ERR_CANT_RESOLVE;