Use unlikely for error treatment.

Use unlikely for error treatment.

When handling error case on SCMallog, SCCalloc or SCStrdup
we are in an unlikely case. This patch adds the unlikely()
expression to indicate this to gcc.

This patch has been obtained via coccinelle. The transformation
is the following:

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

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