Return value of function, which is statistically inferred to return null, but with no source code available, is dereferenced

Defect 104374 (STATIC_C)
Checker NULL_RETURNS (subcategory unimpl)
File: /src/include/ctype.i
Function __wt_isdigit
/src/include/ctype.i, line: 38
Assuming unimplemented function "__ctype_b_loc" may return a null value.

    	return (isdigit(c) != 0);

/src/include/ctype.i, line: 38
Dereferencing "__ctype_b_loc()", which is known to be "NULL".

    	return (isdigit(c) != 0);