Added some null pointer checks.

Added a design doc.

design.md

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+Design notes for source code
+==
+This file give some explanations and guidelines regarding ModSecurity v2 source code.
+The goal is to discuss topics that are not related to a specific location in the code, so that cannot be best explained by comments.
+The goal is not to replace comments where it is probably better.
+It's quite short for the moment, but the goal is to extend it from time to time.
+
+## Null pointer check
+The default behaviour is to check for null pointer dereference everywhere it may be needed.
+In case a pointer cannot be null, it has to be explained with a comment at the beginning of the function of when dereferencing the pointer.
+On top of that, an explicit check should be done when compiling in debug mode with the following code:
+```
+    assert(mypointer);
+```
+In case a pointer that cannot be null is used at several locations (say more than 3 times),
+the explanation could be given globally in this file.
+
+### Pointers never null
+The following pointers can never be null:
+
+#### msr
+
+msr is assigned at the following places:
+- mod_security2.c (14 x): initialization
+In all the above calls, and all calling functions, it immediately returns (with an error code) in case msr is null, up to a place where no mod_security2 processing at all occurs.
+In subsequent calls, there's thus no possibility to have msr null.
+- apache2_io.c (2 x): assign a previously initialized msr
+- msc_json (9 x):     assign a previously initialized msr
+- msc_lua.c (4 x):    assign a previously initialized msr