Crash was seen on recent image with this BT on top of the stack

(gdb) bt full
(mif=0x7fffb6226568) at
/vpp/build-data/../src/plugins/memif/memif.c:297
ring = 0x0 <<<<<<<<<<
i = 0
j = 0
buffer_offset = 65792
r = 0x7fffb5e59f80
alloc = {flags = 1, name = 0x7fffb449f965 "memif region",
size = 4260096, numa_node = 0, addr = 0x7fff41dac000,
fd = 11,
log2_page_size = 12, n_pages = 1041}
err = 0x0
_FUNCTION_ = "memif_init_regions_and_queues"

The crash happened at this line.
ring = memif_get_ring (mif, MEMIF_RING_S2M, i);
ring=>head = ring->tail = 0; <=====

Please note that the crash is caused by dereferencing NULL rinng.
Put breakpoint into the function. I notice that
mif->regions[0].shm is not initialized.

(gdb) p mif->regions[0].shm
$8 = (void *) 0x0

It looks like we forgot to set shm after clib_mem_vm_ext_alloc().
Add the missing cide and the crash is fixed.

Change-Id: Ib722a6c241c77acfa8e33962106b57faa50e1ea7
Signed-off-by: Steven <sluong@cisco.com>
(cherry picked from commit 9fefa9a697daf0e949ea7a2700ecaf2ba4d1d2cb)