decoder: Fix BININT decoding for negative values

Found via fuzzing:

	"I-7\n."

	panic: protocol 1: decode·encode != identity:
	have: 4294967289
	want: -7

	goroutine 1 [running]:
	github.com/kisielk/og-rek.Fuzz(0x7f99bd8b4000, 0x5, 0x200000, 0x3)
	        /tmp/go-fuzz-build914098789/gopath/src/github.com/kisielk/og-rek/fuzz.go:50 +0x604
	go-fuzz-dep.Main(0x524df8)
	        /tmp/go-fuzz-build914098789/goroot/src/go-fuzz-dep/main.go:49 +0xad
	main.main()
	        /tmp/go-fuzz-build914098789/gopath/src/github.com/kisielk/og-rek/go.fuzz.main/main.go:10 +0x2d
	exit status 2

I've checked other handlers, like BININT1 and BININT2, and since there
everywhere argument is unsigned, there is no similar problem.

We needed previous patch on proper readLine EOF detection, because else
the testcase for P0("I-7\n.") would be breaking:

    --- FAIL: TestDecode/int(-7)/"I-7\n." (0.00s)
        ogorek_test.go:401: no ErrUnexpectedEOF on [:2] truncated stream: v = <nil>  err = &strconv.NumError{Func:"ParseInt", Num:"-", Err:(*errors.errorString)(0xc00000e1b0)}

ogorek.go

@@ -483,7 +483,7 @@ func (d *Decoder) loadBinInt() error {
 		return err
 	}
 	v := binary.LittleEndian.Uint32(b[:])
-	d.push(int64(v))
+	d.push(int64(int32(v))) // NOTE signed: uint32 -> int32, and only then -> int64
 	return nil
 }
 

ogorek_test.go

@@ -174,6 +174,10 @@ var tests = []TestEntry{
 		P0("I74565\n."),            // INT
 		P1_("J\x45\x23\x01\x00.")), // BININT
 
+	X("int(-7)", int64(-7),
+		P0("I-7\n."),               // INT
+		P1_("J\xf9\xff\xff\xff.")), // BININT
+
 	X("float", float64(1.23),
 		P0("F1.23\n."),                    // FLOAT
 		P1_("G?\xf3\xae\x14z\xe1G\xae.")), // BINFLOAT