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/00opcode_uc_crash.c

@@ -0,0 +1,69 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+
+#include <unicorn/unicorn.h>
+
+#define X86_CODE32 "\x00" // add byte ptr ds:[eax],al
+#define ADDRESS 0x1000000
+
+static void VM_exec()
+{
+    uc_engine *uc;
+    uc_err err;
+    uint32_t tmp;
+    unsigned int r_eax;
+
+    r_eax = 0x1000008;
+
+    // Initialize emulator in X86-32bit mode
+    err = uc_open(UC_ARCH_X86, UC_MODE_32, &uc);
+    if(err)
+    {
+        printf("Failed on uc_open() with error returned: %s", uc_strerror(err));
+        return;
+    }
+
+    err = uc_mem_map(uc, ADDRESS, (4 * 1024 * 1024), UC_PROT_ALL);
+    if(err != UC_ERR_OK)
+    {
+        printf("Failed to map memory %s", uc_strerror(err));
+        return;
+    }
+
+    // write machine code to be emulated to memory
+    err = uc_mem_write(uc, ADDRESS, X86_CODE32, sizeof(X86_CODE32) - 1);
+    if(err != UC_ERR_OK)
+    {
+        printf("Failed to write emulation code to memory, quit!: %s(len %d)", uc_strerror(err), sizeof(X86_CODE32) - 1);
+        return;
+    }
+
+    // initialize machine registers
+    uc_reg_write(uc, UC_X86_REG_EAX, &r_eax);
+
+    // emulate machine code in infinite time
+    err = uc_emu_start(uc, ADDRESS, ADDRESS + (sizeof(X86_CODE32) - 1), 0, 0);
+    if(err)
+    {
+        printf("Failed on uc_emu_start() with error returned %u: %s", err, uc_strerror(err));
+
+        uc_close(uc);
+        return;
+    }
+
+    if (!uc_mem_read(uc, ADDRESS+8, &tmp, sizeof(tmp)))
+        printf(">>> Read 4 bytes from [0x%08X] = 0x%08X\n", ADDRESS+8, tmp); //should contain the byte '8'
+    else
+        printf(">>> Failed to read 4 bytes from [0x%08X]\n", ADDRESS+8);
+
+    uc_close(uc);
+
+    puts("No crash. Yay!");
+}
+
+int main(int argc, char *argv[])
+{
+    VM_exec();
+    return 0;
+}

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)