In UTF-16[BE|LE] decoder with last=true, check space without intent t…

…o write if the decoder has pending state.

This ensures that the outer caller has space for the REPLACEMENT CHARACTER
as happens more naturally with other encodings.

Closes #44.

src/utf_16.rs

@@ -90,24 +90,31 @@ impl Utf16Decoder {
         },
         {
             debug_assert!(!self.pending_bmp);
-            if self.lead_surrogate != 0 {
-                self.lead_surrogate = 0;
-                match self.lead_byte {
-                    None => {
-                        return (DecoderResult::Malformed(2, 0), src_consumed, dest.written());
+            if self.lead_surrogate != 0 || self.lead_byte.is_some() {
+                // We need to check space without intent to write in order to
+                // make sure that there is space for the replacement character.
+                match dest.check_space_bmp() {
+                    Space::Full(_) => {
+                        return (DecoderResult::OutputFull, 0, 0);
                     }
-                    Some(_) => {
+                    Space::Available(_) => {
+                        if self.lead_surrogate != 0 {
+                            self.lead_surrogate = 0;
+                            match self.lead_byte {
+                                None => {
+                                    return (DecoderResult::Malformed(2, 0), src_consumed, dest.written());
+                                }
+                                Some(_) => {
+                                    self.lead_byte = None;
+                                    return (DecoderResult::Malformed(3, 0), src_consumed, dest.written());
+                                }
+                            }
+                        }
+                        debug_assert!(self.lead_byte.is_some());
                         self.lead_byte = None;
-                        return (DecoderResult::Malformed(3, 0), src_consumed, dest.written());
+                        return (DecoderResult::Malformed(1, 0), src_consumed, dest.written());
                     }
-                }
-            }
-            match self.lead_byte {
-                None => {}
-                Some(_) => {
-                    self.lead_byte = None;
-                    return (DecoderResult::Malformed(1, 0), src_consumed, dest.written());
-                }
+                }                
             }
         },
         {