r.stats: Fix Resource Leak issue in raw_stats.c

[ShubhamDesai]

This pull request fixes issue identified by Coverity Scan (CID : 1207664, 1207665, 1207666).
Used G_free() to fix this issue.

[nilason]

rast and rastp are both an array of array (void**), you need to deal with that.
(One of them is tricky, I'll leave you to solve that :)

[ShubhamDesai]

rast and rastp are both an array of array (void**), you need to deal with that. (One of them is tricky, I'll leave you to solve that :)

Done

For rast: Since each rast[i] holds a buffer for a raster row (allocated with Rast_allocate_buf), I added a loop to call G_free(rast[i]) for each one. After that, I G_free(rast) to release the array of pointers itself.

For rastp : While rastp is an array of pointers itself (so I G_free(rastp)), its individual pointers (rastp[i]) actually just point into the buffers that rast[i] owns. So, there's no need to free rastp[i] individually, as that memory is already handled when rast[i] is freed.