C#: Fix false positives in cs/dereferenced-value-may-be-null

csharp/ql/src/semmle/code/csharp/controlflow/Guards.qll

@@ -578,8 +578,12 @@ module Internal {
    */
   Expr getNullEquivParent(Expr e) {
     result = any(QualifiableExpr qe |
-      qe.getQualifier() = e and
       qe.isConditional() and
+      (
+        e = qe.getQualifier()
+        or
+        e = qe.(ExtensionMethodCall).getArgument(0)
+      ) and
       (
         // The accessed declaration must have a value type in order
         // for `only if` to hold

csharp/ql/src/semmle/code/csharp/dataflow/Nullness.qll

@@ -155,6 +155,9 @@ private predicate defMaybeNull(Ssa::Definition def, string msg, Element reason)
     reason = de.getANullCheck(_, true) and
     msg = "as suggested by $@ null check" and
     not def instanceof Ssa::PseudoDefinition and
+    strictcount(Location l |
+      l = any(Ssa::Definition def0 | de = def0.getARead()).getLocation()
+    ) = 1 and
     not nonNullDef(def) and
     // Don't use a check as reason if there is a `null` assignment
     not def.(Ssa::ExplicitDefinition).getADefinition().getSource() instanceof MaybeNullExpr