MINOR: chunks: always initialize the output chunk in get_trash_chunk()

The get_trash_chunk() function is convenient and is sometimes used even
to get a temporary string. While the chunk is initialized, the string
may contain some random garbage that some code might retrieve if it uses
chunk->str directly without checking ->len. This is what happened in checks
after commit 25e2ab5 (MEDIUM: checks: centralize error reporting). It's not
easy to guess it at first so better pre-initialize the string with a zero.

src/chunk.c

@@ -33,7 +33,9 @@ static char *trash_buf2;
 * type of conversion. Two chunks and their respective buffers are alternatively
 * returned so that it is always possible to iterate data transformations without
 * losing the data being transformed. The blocks are initialized to the size of
-* a standard buffer, so they should be enough for everything.
+* a standard buffer, so they should be enough for everything. For convenience,
+* a zero is always emitted at the beginning of the string so that it may be
+* used as an empty string as well.
 */
 struct chunk *get_trash_chunk(void)
 {
@@ -47,6 +49,7 @@ struct chunk *get_trash_chunk(void)
 		trash_chunk = &trash_chunk1;
 		trash_buf = trash_buf1;
 	}
+	*trash_buf = 0;
 	chunk_init(trash_chunk, trash_buf, trash_size);
 	return trash_chunk;
 }