[lldb] Fix nullptr dereference on running x86 binary with x86-disabled llvm (#82603)

If `LLVM_TARGETS_TO_BUILD` does not contain `X86` and we try to run an
x86 binary in lldb, we get a `nullptr` dereference in
`LLVMDisasmInstruction(...)`. We try to call `getDisAsm()` method on a
`LLVMDisasmContext *DC` which is null. The pointer is passed from
`x86AssemblyInspectionEngine::instruction_length(...)` and is originally
`m_disasm_context` member of `x86AssemblyInspectionEngine`. This should
be filled by `LLVMCreateDisasm(...)` in the class constructor, but not
having X86 target enabled in llvm makes
`TargetRegistry::lookupTarget(...)` call return `nullptr`, which results
in `m_disasm_context` initialized with `nullptr` as well.

This patch adds if statements against `m_disasm_context` in
`x86AssemblyInspectionEngine::GetNonCallSiteUnwindPlanFromAssembly(...)`
and `x86AssemblyInspectionEngine::FindFirstNonPrologueInstruction(...)`
so subsequent calls to
`x86AssemblyInspectionEngine::instruction_length(...)` do not cause a
null pointer dereference.