Bug#43932 myisam index corruption with large index and large

key_buffer_size.

The cause of corruption was number overflow when multiplying 
two ulong values, number of used keycache blocks with size
of a single block. The result of multiplication exceeded ulong 
range (4G) and this lead to incorrectly calculated  buffer offset
in the key cache.

The fix is to use size_t for multiplication result.

This patch also fixes pointless cast in safemalloc 
(size of allocated block to uint), that creates lot of false
alarm warnings when using big keycache (> 4GB) in debug mode.

mysys/mf_keycache.c

@@ -2044,13 +2044,15 @@ restart:
         }
         else
         {
+          size_t block_mem_offset;
           /* There are some never used blocks, take first of them */
           DBUG_ASSERT(keycache->blocks_used <
                       (ulong) keycache->disk_blocks);
           block= &keycache->block_root[keycache->blocks_used];
+          block_mem_offset= 
+           ((size_t) keycache->blocks_used) * keycache->key_cache_block_size;
           block->buffer= ADD_TO_PTR(keycache->block_mem,
-                                    ((ulong) keycache->blocks_used*
-                                     keycache->key_cache_block_size),
+                                    block_mem_offset,
                                     uchar*);
           keycache->blocks_used++;
           DBUG_ASSERT(!block->next_used);

mysys/safemalloc.c

@@ -174,7 +174,7 @@ void *_mymalloc(size_t size, const char *filename, uint lineno, myf MyFlags)
   data[size + 3]= MAGICEND3;
   irem->filename= (char *) filename;
   irem->linenum= lineno;
-  irem->datasize= (uint32) size;
+  irem->datasize= size;
   irem->prev=	  NULL;
 
   /* Add this remember structure to the linked list */