In __mbsconv(), if prec was zero, nconv could have been used uninitialized.  Initialize it to a safe value so that there's no chance of returning an error if stack garbage happens to be equal to (size_t)-1 or (size_t)-2.
In __mbsconv(), if prec was zero, nconv could have been used
uninitialized.  Initialize it to a safe value so that there's no
chance of returning an error if stack garbage happens to be equal to
(size_t)-1 or (size_t)-2.

Found by:	Clang static analyzer
MFC after:	7 days

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