On 64 bit system, the byte size of the following struct is 16:
    type myStruct struct {
       a int
       b uint32
    }
But extractVarInfoFromEntry only allocates a mem of 12 bytes for it.
When calling method of this struct with the "call" command, it will
result in this error:
    write out of bounds

This patch extends the mem by adding padding bytes to the end of the
mem.

Fixes go-delve#3364.