Fix logmailer problem for dynamic lib use of glog.

[guoyuhong]

The problem I met.

I want to use glog in a dynamic lib (.so file), e.g. use it with Apache/Arrow.
However, if there is a global variable defined in non-empty std::string, there will be memory problems. I can catch this problem by valgrind. The following picture is the valgrind log.

PR change

I have changed the default value of logmailer to an empty std::string and put the default value to where FLAGS_logmailer is used when it is empty.

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