patch 8.0.0207: leaking file descriptor when system() fails

Problem:    Leaking file descriptor when system() cannot find the buffer.
            (Coverity)
Solution:   Close the file descriptor.  (Dominique Pelle, closes #1398)

src/evalfunc.c

@@ -11832,6 +11832,7 @@ get_cmd_output_as_rettv(
 	    if (buf == NULL)
 	    {
 		EMSGN(_(e_nobufnr), argvars[1].vval.v_number);
+		fclose(fd);
 		goto errret;
 	    }