Add fuzz target for FromString

Uses the fuzzer of the Go tool chain in [branch
dev.fuzz](https://github.com/golang/go/tree/dev.fuzz).

See also golang/go#44551 and https://go.googlesource.com/proposal/+/master/design/draft-fuzzing.md

airac_fuzz_test.go

@@ -0,0 +1,30 @@
+//+build go.dev.fuzz
+
+package airac
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+)
+
+//goland:noinspection GoUnusedExportedFunction
+func FuzzFromString(f *testing.F) {
+	for i := -999; i <= 9999; i++ {
+		f.Add(strconv.Itoa(i))
+	}
+
+	f.Fuzz(func(t *testing.T, s string) {
+		t.Parallel()
+
+		a, err := FromString(s)
+		if err != nil {
+			t.SkipNow()
+		}
+
+		s2 := a.String()
+		if strings.TrimSpace(s) != s2 {
+			t.Errorf("%q != %q", s, s2)
+		}
+	})
+}

airac_test.go

@@ -238,6 +238,10 @@ func TestFromString(t *testing.T) {
 
 		// found by fuzzer (case testdata/corpus/FuzzFromString/44ea5456f4caf7ee4b0cb896e30cb4f52cd0e65cd3c1843393c5a535f97c7a6e)
 		{"+911", "", 0, 0, false},
+
+
+		// found by fuzzer (case testdata/corpus/FuzzFromString/cde16d0f1e64e24bd633e50af05f0e1c4b74934119ed36156d64e32768cbcc4e)
+		{" 7807", "1978-07-13", 1978, 07, true},
 	}
 
 	for i, tt := range testt {

go.mod

@@ -1,3 +1,3 @@
 module github.com/jwkohnen/airac
 
-go 1.12
+go 1.16

testdata/corpus/FuzzFromString/44ea5456f4caf7ee4b0cb896e30cb4f52cd0e65cd3c1843393c5a535f97c7a6e

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("+911")

testdata/corpus/FuzzFromString/cde16d0f1e64e24bd633e50af05f0e1c4b74934119ed36156d64e32768cbcc4e

@@ -0,0 +1,2 @@
+go test fuzz v1
+string(" 7807")