easy: resize receive buffer on easy handle reset

- In curl_easy_reset attempt to resize the receive buffer to its default
  size. If realloc fails then continue using the previous size.

Prior to this change curl_easy_reset did not properly handle resetting
the receive buffer (data->state.buffer). It reset the variable holding
its size (data->set.buffer_size) to the default size (READBUFFER_SIZE)
but then did not actually resize the buffer. If a user resized the
buffer by using CURLOPT_BUFFERSIZE to set the size smaller than the
default, later called curl_easy_reset and attempted to reuse the handle
then a heap overflow would very likely occur during that handle's next
transfer.

Reported-by: Felix Hädicke

Fixes https://github.com/curl/curl/issues/4143
Closes https://github.com/curl/curl/pull/4145

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