Fix issue with error function vars

Fix false positive for this case:
```go
It("should not trigger warning for nil function vars", func() {
	f := func(string) error { return nil }

	f = nil
	Expect(f).To(BeNil()) // <== should not trigger a warning, but it did
})
```

analyzer_test.go

@@ -97,6 +97,10 @@ func TestAllUseCases(t *testing.T) {
 			testName: "HaveOccurred matcher tests",
 			testData: "a/haveoccurred",
 		},
+		{
+			testName: "nil error-func variable",
+			testData: "a/issue-171",
+		},
 	} {
 		t.Run(tc.testName, func(tt *testing.T) {
 			analysistest.Run(tt, analysistest.TestData(), ginkgolinter.NewAnalyzer(), tc.testData)

internal/rules/errorequalnilrule.go

@@ -12,8 +12,15 @@ import (
 type ErrorEqualNilRule struct{}
 
 func (ErrorEqualNilRule) isApplied(gexp *expression.GomegaExpression, config types.Config) bool {
-	return !bool(config.SuppressErr) &&
-		gexp.ActualArgTypeIs(actual.ErrorTypeArgType) &&
+	if config.SuppressErr {
+		return false
+	}
+
+	if !gexp.IsAsync() && gexp.ActualArgTypeIs(actual.FuncSigArgType) {
+		return false
+	}
+
+	return gexp.ActualArgTypeIs(actual.ErrorTypeArgType) &&
 		gexp.MatcherTypeIs(matcher.BeNilMatcherType|matcher.EqualNilMatcherType)
 }
 

testdata/src/a/issue-171/issue171.go

@@ -0,0 +1,16 @@
+package issue_171
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("test if issue 171 was solved", func() {
+	It("should not trigger warning for nil function vars", func() {
+		f := func(string) error { return nil }
+
+		f = nil
+
+		Expect(f).To(BeNil())
+	})
+})