Fix a bug that GZipReader#gets may return incomplete line

See also: ruby/csv#117 (comment)

How to reproduce with x.csv.gz in the issue comment:

    Zlib::GzipReader.open("x.csv.gz") do |rio|
      rio.gets(nil, 1024)
      while line = rio.gets(nil, 8192)
        raise line unless line.valid_encoding?
      end
    end

Reported by Dimitrij Denissenko. Thanks!!!

ext/zlib/zlib.c

@@ -4199,17 +4199,17 @@ gzreader_charboundary(struct gzfile *gz, long n)
 {
     char *s = RSTRING_PTR(gz->z.buf);
     char *e = s + ZSTREAM_BUF_FILLED(&gz->z);
-    char *p = rb_enc_left_char_head(s, s + n, e, gz->enc);
+    char *p = rb_enc_left_char_head(s, s + n - 1, e, gz->enc);
     long l = p - s;
     if (l < n) {
-	n = rb_enc_precise_mbclen(p, e, gz->enc);
-	if (MBCLEN_NEEDMORE_P(n)) {
-	    if ((l = gzfile_fill(gz, l + MBCLEN_NEEDMORE_LEN(n))) > 0) {
+	int n_bytes = rb_enc_precise_mbclen(p, e, gz->enc);
+	if (MBCLEN_NEEDMORE_P(n_bytes)) {
+	    if ((l = gzfile_fill(gz, n + MBCLEN_NEEDMORE_LEN(n_bytes))) > 0) {
 		return l;
 	    }
 	}
-	else if (MBCLEN_CHARFOUND_P(n)) {
-	    return l + MBCLEN_CHARFOUND_LEN(n);
+	else if (MBCLEN_CHARFOUND_P(n_bytes)) {
+	    return l + MBCLEN_CHARFOUND_LEN(n_bytes);
 	}
     }
     return n;