[libstdc++ patch] Fix basic_string::_S_construct null pointer check

[Doug Gregor]

basic_string::_S_construct attempts to do a null-pointer check by 
comparing the "__beg" iterator to a default-constructed iterator of the 
same type. This both triggers an error in debug mode ("attempt to 
compare against a singular iterator") and could cause incorrect 
behavior in release mode in the following unlikely circumstance:

	1) [begin, end) is an empty iterator range, and
	2) end is equal to a default-constructed iterator, and
	3) the user passes in an allocator that is not equivalent to a 
default-constructed allocator

Tested on powerpc-apple-darwin; no regressions in debug or release 
mode. Okay to commit?

	Doug

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