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Include <limits.h>.
(SIZE_MAX): Define if the headers do not.
(verror): Do not give up if vsnprintf returns a negative count.
Instead, grow the buffer.  This ports to Windows vsnprintf, which
does not conform to C99.  Problem reported by Eli Zaretskii.
Also, simplify the allocation scheme, by avoiding the need for
calling realloc, and removing the ALLOCATED variable.