Fix issue #4001: invalid pragma {. hint[]: off .}

The bug was in processNote, module pragmas. The code assumed that
a nkBracketExpr AST node always had two children (without testing
this), and tried to access elements with index 0 and 1 in the sons
array of the nkBracketExpr node.

The code that triggered the bug was just {. hint[]: off .}
by itself in a module; in this case the nkBracketExpr has only
one children in the sons array, so the code in processNote caused
an out-of-bounds array access.

This commit also adds a test to guarantee that this pragma is
rejected, as is.

compiler/pragmas.nim

@@ -277,6 +277,7 @@ proc processDynLib(c: PContext, n: PNode, sym: PSym) =
 proc processNote(c: PContext, n: PNode) =
   if (n.kind == nkExprColonExpr) and (sonsLen(n) == 2) and
       (n.sons[0].kind == nkBracketExpr) and
+      (n.sons[0].sons.len == 2) and
       (n.sons[0].sons[1].kind == nkIdent) and
       (n.sons[0].sons[0].kind == nkIdent):
       #and (n.sons[1].kind == nkIdent):

tests/pragmas/thintoff.nim

@@ -0,0 +1,6 @@
+# issue #4001
+discard """
+  errormsg: "invalid pragma: hint[]: off"
+"""
+
+{. hint[]: off .}