Fix panic when string field occurs more than once

Also add test

Thanks to `cargo fuzz` which revealed this problem

protobuf-test/src/common/v2/test_singular_concat.rs

@@ -0,0 +1,21 @@
+//! Test when two messages are concatenated, singular fields are taken from the last one.
+
+use super::test_singular_concat_pb::*;
+
+use protobuf_test_common::*;
+
+#[test]
+fn test_concat_bytes() {
+    let mut m = TestSingularConcat::new();
+    m.set_b(b"\xdd\xee".to_vec());
+
+    test_deserialize("12 03 aa bb cc 12 02 dd ee", &m);
+}
+
+#[test]
+fn test_concat_string() {
+    let mut m = TestSingularConcat::new();
+    m.set_s("\x61\x62".to_string());
+
+    test_deserialize("0a 03 21 22 23 0a 02 61 62", &m);
+}

protobuf-test/src/common/v2/test_singular_concat_pb.proto

@@ -0,0 +1,8 @@
+syntax = "proto2";
+
+package test_singular_concar;
+
+message TestSingularConcat {
+    optional string s = 1;
+    optional bytes b = 2;
+}

protobuf/src/stream.rs

@@ -701,8 +701,7 @@ impl<'a> CodedInputStream<'a> {
     }
 
     pub fn read_string_into(&mut self, target: &mut String) -> ProtobufResult<()> {
-        // assert string is empty, otherwize UTF-8 validation is too expensive
-        assert!(target.is_empty());
+        target.clear();
         // take target's buffer
         let mut vec = mem::replace(target, String::new()).into_bytes();
         self.read_bytes_into(&mut vec)?;