Fix +nnn bug in FromString

While doing experiments with the dev.fuzz branch of the Go tool chain,
I've found a bug in FromString() within 1 second of fuzz time:

Because I don't use a regexp in the parseIdentifier, but instead simply
use strconv.Atoi, it is possible to sneak in a sign symbol.  `-` is
detected, but + is not.  There is not much harm, as an AIRAC identifier
like "+911" is interpreted as "0911", but technically it is not a valid
identifier and should be rejected.  Also, although a `-` sign correctly
yields errors, it is not clear in the code, why.

This change rejects + and - signs explicitly.

airac.go

@@ -93,6 +93,11 @@ func parseIdentifier(yyoo string) (year, ordinal int, err error) {
 	if len(yyoo) != 4 {
 		return 0, 0, fmt.Errorf("illegal AIRAC id %q", yyoo)
 	}
+
+	if sign := yyoo[0]; sign == '+' || sign == '-' {
+		return 0, 0, fmt.Errorf("illegal AIRAC id %q", yyoo)
+	}
+
 	yyooInt, err := strconv.Atoi(yyoo)
 	if err != nil {
 		return 0, 0, fmt.Errorf("illegal AIRAC id %q", yyoo)

airac_test.go

@@ -235,6 +235,9 @@ func TestFromString(t *testing.T) {
 		{"a", "", 0, 0, false},
 		{"aa", "", 0, 0, false},
 		{"", "", 0, 0, false},
+
+		// found by fuzzer (case testdata/corpus/FuzzFromString/44ea5456f4caf7ee4b0cb896e30cb4f52cd0e65cd3c1843393c5a535f97c7a6e)
+		{"+911", "", 0, 0, false},
 	}
 
 	for i, tt := range testt {