internal/encoding/text: fix parsing of incomplete numbers

Fix a panic when parsing the incomplete negative number "- ".

Fixes golang/protobuf#1530

Change-Id: Iba5e8ee68d5f7255c28f1a74f31beee36c9ed847
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/475995
Reviewed-by: Lasse Folger <lassefolger@google.com>
Run-TryBot: Damien Neil <dneil@google.com>

encoding/prototext/decode_test.go

@@ -308,6 +308,11 @@ s_string: "谷歌"
 		inputMessage: &pb3.Scalars{},
 		inputText:    "s_sfixed64: bad",
 		wantErr:      "invalid value for sfixed64",
+	}, {
+		desc:         "incomplete number value",
+		inputMessage: &pb3.Scalars{},
+		inputText:    `s_int32: - `,
+		wantErr:      "(line 1:10): invalid scalar value: -",
 	}, {
 		desc:         "conformance: FloatFieldMaxValue",
 		inputMessage: &pb2.Scalars{},

internal/encoding/text/decode_number.go

@@ -88,15 +88,15 @@ func parseNumber(input []byte) number {
 		neg = true
 		s = s[1:]
 		size++
-		if len(s) == 0 {
-			return number{}
-		}
 		// Consume any whitespace or comments between the
 		// negative sign and the rest of the number
 		lenBefore := len(s)
 		s = consume(s, 0)
 		sep = lenBefore - len(s)
 		size += sep
+		if len(s) == 0 {
+			return number{}
+		}
 	}
 
 	switch {

internal/encoding/text/decode_test.go

@@ -559,6 +559,14 @@ func TestDecoder(t *testing.T) {
 			in:   "- \t 123.321e6",
 			want: []R{{E: "invalid field number: -123.321e6"}},
 		},
+		{
+			in:   "-",
+			want: []R{{E: "invalid field name: -"}},
+		},
+		{
+			in:   "- ",
+			want: []R{{E: "invalid field name: -"}},
+		},
 		{
 			in:   "- # negative\n 123",
 			want: []R{{E: "invalid field number: -123"}},