Issue 719: Refuse to allocate a too-small PPMD7 context

Other parts of the PPMD7 code assume that the dictionary is
at least UNIT_SIZE (12 bytes).  Enforce that assumption here
to avoid potential buffer under- and over-runs.

This was pointed out in a review of the original fix for Issue 719.

libarchive/archive_ppmd7.c

@@ -126,6 +126,11 @@ static Bool Ppmd7_Alloc(CPpmd7 *p, UInt32 size, ISzAlloc *alloc)
 {
   if (p->Base == 0 || p->Size != size)
   {
+    /* RestartModel() below assumes that p->Size >= UNIT_SIZE
+       (see the calculation of m->MinContext). */
+    if (size < UNIT_SIZE) {
+      return False;
+    }
     Ppmd7_Free(p, alloc);
     p->AlignOffset =
       #ifdef PPMD_32BIT