membuffer: be defensive about bad C library vsnprintf implementations

Dirk reports that some Windows users have had odd corruption in the
commit messages in the cloud storage.  They make no sense at all unless
there is some very weird Windows library bug.

The prime suspect is 'vsnprintf()' returning a negative error when the
target buffer is too small (rather than the proper "this is how much
space it would need").  That is a very traditional C library bug that I
thougth had been fixed everywhere, but there doesn't really seem to be a
lot of other likely causes.

So let's make our membuffer code be defensive against bad libraries that
return negative error numbers from vsnprintf.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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