string_parser needs to initialize success flag

[cppljevans]

The string_parser::call needs to initialize the success out value to true.

Here, in the string_parser::call function:

https://github.com/tzlaine/parser/blob/develop/include/boost/parser/parser.hpp#L6913

the only change to the success flag is to false. This is definitely wrong. It needs to be initialized to true at the start of the function. Otherwise, if the success flag is passed as false, it will not change its value, despite the parse being successful.