Prevent GCC from moving a mach_read_from_4() before we have checked that
we have 4 bytes to read. The pointer may only point to a 1, 2 or 3
bytes in which case the code should not read 4 bytes. This is a
workaround to a GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77673

Patch submitted by: Laurynas Biveinis <laurynas.biveinis@gmail.com>
RB: 14135
Reviewed by: Pawel Olchawa <pawel.olchawa@oracle.com>