Fix use-after-free in Reader::parse(std::istream&)

[baylesj]

The istream overload stored the document in a local String then
passed raw pointers into it to parse(const char*, const char*),
which kept those pointers in begin_/end_. After parse() returned
the local String was destroyed, leaving begin_/end_ dangling.
Any subsequent call to getFormattedErrorMessages() would then
read freed memory.

Fix by reading the stream into the member document_ instead, matching
the behavior of parse(const std::string&).

Also document the lifetime requirement on parse(const char*, const
char*): the caller's buffer must outlive the Reader if error-reporting
methods are used after parsing.

Fixes #1623

[coveralls]

Pull Request Test Coverage Report for Build 23129789951

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.093%

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Totals
Change from base Build 23129754687:	0.0%
Covered Lines:	2574
Relevant Lines:	2691

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💛 - Coveralls