malloc.c: check allocation sizes against PTRDIFF_MAX.

I don't expect this to actually come up in any circumstance, but it
prevents a warning in some versions of gcc that would otherwise arise
from the use of 'int' to compute the input size: if gcc isn't
confident that the int is positive, then it complains that possible
inputs to malloc might be in the region of 2^64 - (small multiple of a
negative 32-bit int).

I would hope malloc would fail in any case on such an input, so
failing a couple of lines earlier makes no important difference.

Annoyingly, stdint.h is missing in my NestedVM build setup (though it
has stdbool.h - it's not _totally_ C90). So I have to check that at
cmake time.

Also, removed the #defines for smalloc and friends from the tree234
test mode. These were needed in the old build system, when
tree234-test was built ad-hoc without being linked against malloc.c.
But now tree234-test links against the same utils library as
everything else, and can use the real smalloc - and doing so prevents
another of these warnings when compiling with -flto.

tree234.c

@@ -43,9 +43,6 @@ static void logprintf(const char *fmt, ...)
     va_end(ap);
 }
 #define LOG(x) (logprintf x)
-#define smalloc malloc
-#define srealloc realloc
-#define sfree free
 #else
 #define LOG(x)
 #endif