patch 8.0.0598: building with gcc 7.1 yields new warnings

Problem:    Building with gcc 7.1 yields new warnings.
Solution:   Initialize result. (John Marriott)

src/ex_docmd.c

@@ -10890,6 +10890,9 @@ eval_vars(
 		result = strbuf;
 		break;
 #endif
+	default:
+		result = (char_u *)""; /* avoid gcc warning */
+		break;
 	}
 
 	resultlen = (int)STRLEN(result);	/* length of new string */