tool_cb_wrt: fix invalid unicode for windows console

- Suppress an incomplete UTF-8 sequence at the end of the buffer.

- Attempt to reconstruct incomplete UTF-8 sequence from prior call(s)
  in current call.

Prior to this change, in Windows console UTF-8 sequences split between
two or more calls to the write callback would cause invalid "replacement
characters" U+FFFD to be printed instead of the actual Unicode
character. This is because in Windows only UTF-16 encoded characters are
printed to the console, therefore we convert the UTF-8 contents to
UTF-16, which cannot be done with partial UTF-8 sequences.

Reported-by: Maksim Arhipov

Fixes https://github.com/curl/curl/issues/9841
Closes https://github.com/curl/curl/pull/10890

src/tool_operate.c

@@ -464,6 +464,12 @@ static CURLcode post_per_transfer(struct GlobalConfig *global,
     }
   }
 
+#ifdef WIN32
+  /* Discard incomplete UTF-8 sequence buffered from body */
+  if(outs->utf8seq[0])
+    memset(outs->utf8seq, 0, sizeof(outs->utf8seq));
+#endif
+
   /* if retry-max-time is non-zero, make sure we haven't exceeded the
      time */
   if(per->retry_numretries &&