[flang] Fix -Wsign-compare in check-call.cpp (NFC)

/data/llvm-project/flang/lib/Semantics/check-call.cpp:1234:29: error: comparison of integers of different signs: 'int' and 'size_type' (aka 'unsigned long') [-Werror,-Wsign-compare]
  CHECK(index >= 0 && index < actuals.size());
                      ~~~~~ ^ ~~~~~~~~~~~~~~
/data/llvm-project/flang/include/flang/Common/idioms.h:89:20: note: expanded from macro 'CHECK'
                   ^
1 error generated.