Bug fix for buddy allocator

[eugene536]

There are two corner cases that are not handled properly and as a result, you can get panic in bd_init.

First minimal example:

int sz = 256;
void *mem = malloc(sz);
bd_init(mem, mem + sz);

In bd_initfree for k = 0 we will have left = 11 and right = 16 but we have only 16 blocks for k = 0 and maximum allowed index to access in array bd_sizes[0].alloc(in function bd_initfree_pair) is 15. Therefore we have index out of range (but we will never find it with AddressSanitizer) and we will just push the last block to list and calculate wrongly free space.

Second example:

int sz = 224;
void *mem = malloc(sz);
bd_init(mem, mem + sz);

For k = 1 we will have 8 blocks at size 32 bytes, but the first 6 blocks and the last one will be marked as alloc. In function bd_initfree we will have left = 6 and right = 7 as a result we will add sixth block to the free list twice and calculate wrongly free space.