[PATCH 1/1] Analysis: CFG, add comment about NULL CFGBlock successors

[Jiri_Slaby]

CFGBlock successor may be NULL to preserve succs ordering rules. Note
this to the CFGBlock documentation.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>

[Ted_Kremenek]

committed in r130387. Thanks!