Fix infinite recursion bug when using zlib output compression.

Cause: the chunk size is taken from the zlib.output_compression setting,
which is 0 or 1.  This causes the block_size for output buffer to be set
to 0 (1 / 2) and thus causes infinite recursion in php_ob_allocate().
Solution: use a value of 0 for the chunk size which will use the default
sizes.  Also add a sanity check which will default the block_size to 1
if it ends up as 0.

ext/zlib/zlib.c

@@ -230,6 +230,8 @@ PHP_RINIT_FUNCTION(zlib)
 	ZLIBG(ob_gzhandler_status) = 0;
 	ZLIBG(ob_gzip_coding) = 0;
 	if (chunk_size) {
+		if (chunk_size == 1)
+			chunk_size = 0; /* use the default size */
 		php_enable_output_compression(chunk_size TSRMLS_CC);
 	}
 	return SUCCESS;

main/output.c

@@ -132,6 +132,8 @@ PHPAPI int php_start_ob_buffer(zval *output_handler, uint chunk_size, zend_bool
 	if (chunk_size) {
 		initial_size = (chunk_size*3/2);
 		block_size = chunk_size/2;
+		if (block_size == 0)
+			block_size = 1;
 	} else {
 		initial_size = 40*1024;
 		block_size = 10*1024;