fix first chunk substring search

aiohttp/multipart.py

@@ -305,14 +305,18 @@ def _read_chunk_from_stream(self, size):
         """
         assert size >= len(self._boundary) + 2, \
             'Chunk size must be greater or equal than boundary length + 2'
-        if self._prev_chunk is None:
+        first_chunk = self._prev_chunk is None
+        if first_chunk:
             self._prev_chunk = yield from self._content.read(size)
 
         chunk = yield from self._content.read(size)
 
         window = self._prev_chunk + chunk
         sub = b'\r\n' + self._boundary
-        idx = window.find(sub, len(self._prev_chunk) - len(sub))
+        if first_chunk:
+            idx = window.find(sub)
+        else:
+            idx = window.find(sub, len(self._prev_chunk) - len(sub))
         if idx >= 0:
             # pushing boundary back to content
             self._content.unread_data(window[idx:])