Set eof to true at the end of a FlateStream

TL;DR:  This commit fixes issue #3885 by setting `this.eof` to `true` whenever the check inside `readBlock` that is responsible for handling this is not run because the loop containing this check is not executed.

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At the initialization of `Lexer_getObj` (in `parser.js`), there's a loop that skips whitespace and breaks out whenever EOF is encountered. (https://github.com/mozilla/pdf.js/blob/88ec2bd1a/src/core/parser.js#L586-L599)

Whenever the current character is not a whitespace character, `ch = this.nextChar();` is used to find the next character (using `return this.currentChar = this.stream.getByte())`).

The aforementioned `getByte` method retrieves the next byte using (https://github.com/mozilla/pdf.js/blob/88ec2bd1a/src/core/stream.js#L122-L128):

      var pos = this.pos;
      while (this.bufferLength <= pos) {
        if (this.eof)
          return -1;
        this.readBlock();
      }
      return this.buffer[this.pos++];

This piece of code relies on this.eof to detect whether the last character has been read. When the stream is a `FlateStream`, and the end of the stream has been reached, then **`this.eof` is not set to `true`**, because this check is done inside a loop that does not occur when the read block size is zero (https://github.com/mozilla/pdf.js/blob/88ec2bd1ac/src/core/stream.js#L511-L517):

      for (var n = bufferLength; n < end; ++n) {
        if (typeof (b = bytes[bytesPos++]) == 'undefined') {
          this.eof = true;
          break;
        }
        buffer[n] = b;
      }

This commit fixes the issue by setting `this.eof` to `true` whenever the loop is not going to run (i.e. when `bufferLength === end`, i.e. `blockLen === 0`).