Use unlikely in malloc failure test.

This patch is a result of applying the following coccinelle
transformation to suricata sources:

  @istested@
  identifier x;
  statement S1;
  identifier func =~ "(SCMalloc|SCStrdup|SCCalloc|SCMallocAligned|SCRealloc)";
  @@

  x = func(...)
  ... when != x
  - if (x == NULL) S1
  + if (unlikely(x == NULL)) S1