[BUG] Get Rows function

[iraj720]

there is a problem when using these kind of .xlsx files
in GetRows funciton it will add all rows and it will allocate huge amount of memory and then it will use only 0:max elements untill GC clocks
makes it unable to run service with limited memory

here is the file to reproduce it :

cost_center_matrix.xlsx

1. add the following lines to (rows.go#L77):
   fmt.Println(max)
   fmt.Println(len(results))
2. read it normally and check the memory usage with this code after reading it:
   var memStats runtime.MemStats
   runtime.ReadMemStats(&memStats)
   fmt.Printf("memory allocated: %d\n", memStats.Alloc)
   runtime.GC()
   runtime.ReadMemStats(&memStats)
   fmt.Printf("memory allocated 2: %d\n", memStats.Alloc)

difference between 2 prints is the unnecessary allocation

https://github.com/qax-os/excelize/blob/master/rows.go#L61-L79

[xuri]

Thanks for your pull request. I have merged this, and this patch will be released in the next version.