Fixed various Undefined Behavior occurrences.

There are a few situations in which Undefined Behavior can occur. This pull request fixes mainly three categories: First,conditionally uninitialized variables. Second missing field initializers. And third the usage of references with va_start, which is not allowed, too. The proposed fixes hopefully improve the reliability and portability.

Conditionally uninitialized:
 - builtin_commandline.cpp:577
 - expand.cpp:869
 - parse_util.cpp:1036

Initialization of POD structs:
 - event.cpp:61
 - autoload.cpp:22

References used with va_start:
 - common.cpp:608

Please have a look at it. Thanks.