checker: disallow deference of nil (fix #25740) (#25746)

Author: kbkpbot

vlib/v/checker/checker.v

@@ -5033,6 +5033,9 @@ fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) ast.Type {
 		if right_type.is_voidptr() {
 			c.error('cannot dereference to void', node.pos)
 		}
+		if right_type == ast.nil_type {
+			c.error('cannot deference a `nil` pointer', node.right.pos())
+		}
 		if mut expr is ast.Ident {
 			if var := expr.scope.find_var('${expr.name}') {
 				if var.expr is ast.UnsafeExpr {

vlib/v/checker/tests/unsafe_deference_nil_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/unsafe_deference_nil_err.vv:1:1: warning: unused variable: `a`
+    1 | a := unsafe { *nil }
+      | ^
+vlib/v/checker/tests/unsafe_deference_nil_err.vv:1:16: error: cannot deference a `nil` pointer
+    1 | a := unsafe { *nil }
+      |                ~~~

vlib/v/checker/tests/unsafe_deference_nil_err.vv

@@ -0,0 +1 @@
+a := unsafe { *nil }