Add some null checks and bounds checks.

Add some null checks and bounds checks.

* Regex.cs (default_startat): New.  Replaces the fragment
'RightToLeft ?  input.Length : 0' with a null-safe variant.
(Replace) <string variant>: Add null checks for argument strings.
(Split): Likewise.
(.ctor): Likewise.  Validate regex options partially.
(Match): Add null check on string arguments, and bounds checks for
offset and length arguments.

svn path=/trunk/mcs/; revision=141457