[spec/function] Specify null dereference behavior for @safe code

Forbid optimizations which assume a null dereference will not occur. `ldc2`
does use those optimizations with `-O2` - see:
https://forum.dlang.org/post/vv6o31$ac9$1@digitalmars.com.

Specify that codegen must detect null dereferences if the system (by
default) does not.
Specify that codegen must detect when any expression causes a null
pointer to be indexed outside the protected first page. Include warning
that dmd does not implement this yet - see
dlang/dmd#17776.

Author: ntrel

spec/function.dd

@@ -4069,15 +4069,15 @@ $(H3 $(LNAME2 safe-values, Safe Values))
 
         $(P A pointer is a safe value when it is one of:)
         $(OL
-            $(LI `null`)
+            $(LI `null` - $(RELATIVE_LINK2 null-dereferences, see below))
             $(LI it points to a memory object that is live and
             the pointed to value in that memory object is safe.)
         )
         $(P Examples:)
         $(SPEC_RUNNABLE_EXAMPLE_RUN
         ---
-        int* n = null; /* n is safe because dereferencing null is a well-defined
-            crash. */
+        int* n = null; /* n is safe because dereferencing null must either crash
+            or abort. */
         int* x = cast(int*) 0xDEADBEEF; /* x is (most likely) unsafe because it
             is not a valid pointer and cannot be dereferenced. */
 
@@ -4169,6 +4169,23 @@ $(H3 $(LNAME2 safe-values, Safe Values))
             expected by the function.)
         )
 
+$(H3 $(LNAME2 null-dereferences, Null Dereferences))
+
+        $(P When generating `@safe` code, a compliant implementation:)
+
+        - Must not assume that a null dereference will not occur. Optimizations
+          that require that assumption cannot be used for `@safe` functions.
+        - Must generate code that will detect and abort execution if a
+          null dereference occurs:
+          - On systems that do not protect access to the first page of memory addresses
+            by default. (Note that e.g. calling
+            [`mprotect`](https://pubs.opengroup.org/onlinepubs/007904875/functions/mprotect.html)
+            is `@system`).
+          - On all systems when an expression causes a null pointer to be indexed,
+            causing a memory access that may be outside the protected first page
+            of memory addresses. $(RED Warning:) $(TT dmd)
+            [has not implemented](https://github.com/dlang/dmd/issues/17776) this yet.
+
 $(H3 $(LNAME2 safe-aliasing, Safe Aliasing))
 
     $(P When one memory location is accessible with two different types, that