SA5011: don't flag indexing of possibly nil slice

Indexing a nil slice doesn't cause a nil pointer dereference, "only"
an out of bounds.

This fix also acts as a workaround for gh-1051 and gh-1053.

Closes gh-1051
Updates gh-1053

(cherry picked from commit 4e580ec)

staticcheck/lint.go

@@ -4099,6 +4099,13 @@ func CheckMaybeNil(pass *analysis.Pass) (interface{}, error) {
 					ptr = instr.Addr
 				case *ir.IndexAddr:
 					ptr = instr.X
+					if _, ok := ptr.Type().Underlying().(*types.Slice); ok {
+						// indexing a nil slice does not cause a nil pointer panic
+						//
+						// Note: This also works around the bad lowering of range loops over slices
+						// (https://github.com/dominikh/go-tools/issues/1053)
+						continue
+					}
 				case *ir.FieldAddr:
 					ptr = instr.X
 				}

staticcheck/testdata/src/CheckMaybeNil/CheckMaybeNil.go

@@ -151,3 +151,36 @@ func fn15(x *int) {
 	assert(x != nil)
 	_ = *x
 }
+
+func fn16() {
+	var xs []int
+	if xs == nil {
+		println()
+	}
+
+	for _, x := range xs {
+		_ = x
+	}
+
+	var xs2 *[1]int
+	if xs2 == nil {
+		println()
+	}
+	for _, x := range xs2 { // want `possible nil pointer dereference`
+		_ = x
+	}
+
+	var xs3 *[]int
+	if xs3 == nil {
+		println()
+	}
+	for _, x := range *xs3 { // want `possible nil pointer dereference`
+		_ = x
+	}
+
+	var xs4 []int
+	if xs4 == nil {
+		println()
+	}
+	_ = xs4[0]
+}