Fix out of bounds access when stdout is empty

lib/std/os/subprocess.c3

@@ -292,6 +292,7 @@ fn String? execute_stdout_to_buffer(char[] buffer, String[] command_line, SubPro
 	SubProcess process = process::create(command_line, options, environment)!;
 	process.join()!;
 	usz len = process.read_stdout(buffer.ptr, buffer.len)!;
+	if (len == 0) return "";
 	return (String)buffer[:len - 1];
 }
 
@@ -529,4 +530,4 @@ fn bool? SubProcess.is_running(&self)
 		self.join()!;
 		return false;
 	$endif
-}
+}