clang/'Dead store': do not assign fnamelen

Vim hasn't changed this line in buf_modname().
Assigning a value to fnamelen is useless
when evaluating strlen() is sufficient for the condition check.

src/nvim/fileio.c

@@ -4280,7 +4280,7 @@ char *modname(const char *fname, const char *ext, bool prepend_dot)
   if (fname == NULL || *fname == NUL) {
     retval = xmalloc(MAXPATHL + extlen + 3);  // +3 for PATHSEP, "_" (Win), NUL
     if (os_dirname((char_u *)retval, MAXPATHL) == FAIL
-        || (fnamelen = strlen(retval)) == 0) {
+        || strlen(retval) == 0) {
       xfree(retval);
       return NULL;
     }