bugzillaId-487: Buffer manager should not use pointer arithmetic on void*

This is imported from bugzilla item: http://bugs.mariadb.com/show_bug.cgi?id=487

martin.brampton@skysql.com 2014-08-27 09:46:21 UTC
In the gwbuf_alloc function, the pointer to the end of the data is computed from the void *start. This is illegal according to the C standard: http://stackoverflow.com/questions/3523145/pointer-arithmetic-for-void-pointer-in-c. Correct use is shown in the gwbuf_clone_portion function.