fix codeql warnings (#3489)

Co-authored-by: Nicole Mazzuca <mazzucan@outlook.com>

stl/inc/charconv

@@ -444,11 +444,10 @@ _NODISCARD inline uint32_t _Bit_scan_reverse(const _Big_integer_flt& _Xval) noex
 
     const uint32_t _Bx = _Xval._Myused - 1;
 
-    _STL_INTERNAL_CHECK(_Xval._Mydata[_Bx] != 0); // _Big_integer_flt should always be trimmed
-
     unsigned long _Index; // Intentionally uninitialized for better codegen
 
-    _BitScanReverse(&_Index, _Xval._Mydata[_Bx]); // assumes _Xval._Mydata[_Bx] != 0
+    _STL_INTERNAL_CHECK(_Xval._Mydata[_Bx] != 0); // _Big_integer_flt should always be trimmed
+    _BitScanReverse(&_Index, _Xval._Mydata[_Bx]); // lgtm [cpp/conditionallyuninitializedvariable]
 
     return _Index + 1 + _Bx * _Big_integer_flt::_Element_bits;
 }

stl/inc/system_error

@@ -579,7 +579,8 @@ public:
             constexpr size_t _Unknown_error_length = sizeof(_Unknown_error) - 1; // TRANSITION, DevCom-906503
             return string{_Unknown_error, _Unknown_error_length};
         } else {
-            return string{_Msg._Str, _Msg._Length};
+            _STL_INTERNAL_CHECK(_Msg._Str != nullptr);
+            return string{_Msg._Str, _Msg._Length}; // lgtm [cpp/uninitializedptrfield]
         }
     }