make sure the argument to ast_malloc() is > 0.

Long explaination:

The behaviour of the underlying malloc(0) differs depending on the
operating system.  Some return NULL (SysV behaviour); some still
allocate a small chunk of memory and return a valid pointer (e.g.
traditional BSD); some (e.g. FreeBSD 6.x) return a non-null pointer
that causes a memory fault if used, even just for reading.

Given the above variety, better never call malloc(0).



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48389 65c4cc65-6c06-0410-ace0-fbb531ad65f3