Explicitly terminate dest string in html_encode

This fixes an issue introduced in commit a00fa5330dd4. As the
destination string produced by html_escape is not explicitly
terminated by the function as introduced in this commit, the
escaped URL in dest continues with characters found in memory
that have not been overwritten, which essentially breaks the
encoded URL. The behaviour prior to commit a00fa5330dd4 was as
follows:

    $ src/gophernicus <<<URL:https://example.com | grep Refresh
      <META HTTP-EQUIV="Refresh" content="1;URL=https://example.com">

The broken behaviour since commit a00fa5330dd4 is this:

    $ src/gophernicus <<<URL:https://example.com | grep Refresh
      <META HTTP-EQUIV="Refresh" content="1;URL=https://example.comtps://example.com"">

Explicitly terminating the destination string by adding a zero
byte seems to fix this; yet, since I am not a C programmer myself,
I may well have missed some ramifications of this change. In any
case, the output produced by gophernicus since commit a00fa5330dd4
is essentially broken, so this is probably still an improvement.

src/gophernicus.c

@@ -127,6 +127,7 @@ void html_encode(const char *unsafe, char *dest, int bufsize)
 			i += 1;
 		}
 	}
+	dest[j] = '\0';
 }
 
 /*