set_global_var() should not pass NULL pointers to dlclose()

Both Coverity and Clang's static analyzer caught this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14044

lib/libzpool/util.c

@@ -228,13 +228,14 @@ set_global_var(char const *arg)
 		fprintf(stderr, "Failed to open libzpool.so to set global "
 		    "variable\n");
 		ret = EIO;
-		goto out_dlclose;
+		goto out_free;
 	}
 
 	ret = 0;
 
 out_dlclose:
 	dlclose(zpoolhdl);
+out_free:
 	free(varname);
 out_ret:
 	return (ret);