git-fast-import possible memory corruption problem

Internal "allocate in bulk, we will never free this memory anyway"
allocator used in fast-import had a logic to round up the size of the
requested memory block in a wrong place (it computed if the available
space is enough to fit the request first, and then carved a chunk of
memory by size rounded up to the alignment, which could go beyond the
actually available space).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -554,6 +554,10 @@ static void *pool_alloc(size_t len)
 	struct mem_pool *p;
 	void *r;
 
+	/* round up to a 'uintmax_t' alignment */
+	if (len & (sizeof(uintmax_t) - 1))
+		len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
+
 	for (p = mem_pool; p; p = p->next_pool)
 		if ((p->end - p->next_free >= len))
 			break;
@@ -572,9 +576,6 @@ static void *pool_alloc(size_t len)
 	}
 
 	r = p->next_free;
-	/* round out to a 'uintmax_t' alignment */
-	if (len & (sizeof(uintmax_t) - 1))
-		len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
 	p->next_free += len;
 	return r;
 }