PR# 14575 Postcondition of sprealloc can be violated on 64-bit system

Based on code inspection only, it appears that one of the postconditions of
`sprealloc' (in malloc.c) can be violated on a 64-bit system.  The postcondition
that seems to be wrong is:

   ENSURE ("Valid new size", (int)(HEADER (object)->ov_size & B_SIZE) >= new_size);

It looks like the cast "(int)" is unnecessary and wrong.  If the size of
the special object exceeds 2^32 (or maybe even 2^31), which is possible on
a 64-bit system, and if sizeof(int) == 4 (it does on Solaris SPARC 64), then
the postcondition will be violated.