Image filter: fixed reading past the received data

Image filter: fixed reading past the received data

The size parser and the image decoders used ctx->length, which is the
allocation size (set to image_filter_buffer when the upstream response
omits Content-Length), as the amount of valid data, so a truncated
response without Content-Length could be parsed or decoded past the
received bytes into uninitialized buffer memory.

The length is now adjusted to the actually-read size once the response
body has been read.

Reported by dukesp69 and YLChen-007.