VPP-152: mheap_alloc returns 0 when the requested heap size is too small

VPP-152: mheap_alloc returns 0 when the requested heap size is too small

mheap_alloc allocates memory_size bytes of memory and returns
a page-aligned memory space prefixed with a word-aligned
header of type mheap_t.

This header includes the actual usable space size, but
when the requested size is too small, the computed size
was incorrect (infinite).

mheap_alloc now returns 0 in such cases.

With help from Yoann Desmouceaux.

Change-Id: I00af63d573d6939aca53dbe7ff612b726bd8f0df
Signed-off-by: Pierre Pfister <ppfister@cisco.com>