imaplib: read/readline: save multipart buffer tail

For resilience if read() or readline() ever complete with more than one
bytes object remaining in the buffer. This is not expected to happen,
but it seems wise to be prepared for a future change making it possible.

Lib/imaplib.py

@@ -359,7 +359,7 @@ def read(self, size):
 
             if len(buf) >= size:
                 parts.append(buf[:size])
-                self._readbuf = [buf[size:]]
+                self._readbuf = [buf[size:]] + self._readbuf[len(parts):]
                 break
             parts.append(buf)
             size -= len(buf)
@@ -389,7 +389,7 @@ def readline(self):
             if pos != -1:
                 pos += 1
                 parts.append(buf[:pos])
-                self._readbuf = [buf[pos:]]
+                self._readbuf = [buf[pos:]] + self._readbuf[len(parts):]
                 break
             parts.append(buf)
             length += len(buf)