coverity/74718: invalid FUNC_ATTR_NONNULL_ARG

- avoid null passed to ELOG format string
- receive (char *) internally
- modify identifier names for consistency
- edit comments for concision and consistency

src/nvim/os_unix.c

@@ -1117,7 +1117,7 @@ int mch_expand_wildcards(int num_pat, char_u **pat, int *num_file,
 
   free(command);
 
-  if (i != 0) {                         /* mch_call_shell() failed */
+  if (i) {                         /* os_call_shell() failed */
     os_remove((char *)tempname);
     free(tempname);
     /*