X86 zero-byte opcode causes Unicorn to crash.

The opcode 0x00 translates as `add byte ptr ds:[eax],al`, which leads to
a segfault in Unicorn.

Using a debugger, I believe the crash is located in
`qemu/translate-all.c` in function `page_flush_tb_1`, more specifically
this code `pd[i].first_tb = NULL;`

tests/regress/Makefile

@@ -13,6 +13,7 @@ TESTS += mem_protect
 TESTS += mem_exec
 TESTS += mips_kseg0_1
 TESTS += eflags_nosync
+TESTS += 00opcode_uc_crash
 
 all: $(TESTS)