[lldb] Fix warnings

This patch fixes:

  lldb/source/Plugins/Instruction/RISCV/EmulateInstructionRISCV.h:34:5:
  error: default label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

and:

  lldb/source/Plugins/Instruction/RISCV/EmulateInstructionRISCV.cpp:194:21:
  error: comparison of integers of different signs: 'int' and 'size_t'
  (aka 'unsigned long') [-Werror,-Wsign-compare]