libdpkg: Make buffer_write buf argument const

lib/dpkg/buffer.c

@@ -96,7 +96,8 @@ buffer_done(buffer_data_t read_data, buffer_data_t write_data)
 }
 
 off_t
-buffer_write(buffer_data_t data, void *buf, off_t length, const char *desc)
+buffer_write(buffer_data_t data, const void *buf, off_t length,
+             const char *desc)
 {
 	off_t ret = length;
 

lib/dpkg/buffer.h

@@ -136,7 +136,7 @@ off_t buffer_copy_IntInt(int i1, int typeIn, int i2, int typeOut,
                          off_t limit, const char *desc,
                          ...) DPKG_ATTR_PRINTF(6);
 
-off_t buffer_write(buffer_data_t data, void *buf,
+off_t buffer_write(buffer_data_t data, const void *buf,
                    off_t length, const char *desc);
 off_t buffer_read(buffer_data_t data, void *buf,
                   off_t length, const char *desc);