fstab-util: detect out-of-range pri= assignments

We would silently ignore them. One would have to be crazy
to do assign an out of range value, but simply ignoring it
bothers me.

src/shared/fstab-util.c

@@ -140,6 +140,9 @@ int fstab_find_pri(const char *options, int *ret) {
         if (r < 0)
                 return r;
 
+        if ((int) pri < 0)
+                return -ERANGE;
+
         *ret = (int) r;
         return 1;
 }