LATX, fix: Avoid invalid IR1 predecessor access

[LaurenIsACoder]

Summary

• Avoid forming ir1_list[-1] when disassembly fails before producing the
  first IR1 instruction.
• Derive the synthesized invalid instruction address from a predecessor only
  after confirming that one exists.
• Keep the touched block compliant with the project indentation rules.

Root cause

The failure path formed a pointer to ir1_list[ir1_num - 1] before checking
whether ir1_num was zero. A decode failure on the first instruction therefore
formed an out-of-bounds predecessor pointer even though the zero-instruction
branch did not dereference it.

Validation

Mac:

• Deterministic RED by temporarily restoring the old hunk: strict checkpatch
  reported 6 errors.
• GREEN: strict checkpatch reports 0 errors and 0 warnings.
• git diff --check passes.
• Standards and specification reviews both pass with no P0-P2 findings.

LoongArch (l1, commit c91ac892ce776d26c2c97fd35c479d5dca1fc16e):

• Incremental compilation of
  libqemu-x86_64-linux-user.fa.p/target_i386_latx_translator_translate.c.o
  passes.
• ninja -C build latx-x86_64 -j8 passes.
• Final ninja -C build -j8 passes.
• meson test -C build --no-rebuild --print-errorlogs: 4/4 tests pass.

Signed-off-by: Wenqiang Wei weiwenqiang@mail.ustc.edu.cn