[llvm] [RISCV] Add missing check before accessing pointer (PR #121816)

[Luke Lau via llvm-commits]

lukel97 wrote:

Since a bunch of people were running into this crash I've cherry-picked the RISCVISelLowering.cpp changes to main in b0e05a5f04d3225b8ff1319064c88d0249e683d2, but it still needs the test case from this PR

https://github.com/llvm/llvm-project/pull/121816