Fix: ipc: Correctly compare values for the size of ipc buffer and prevent suggesting a negative value when it's insufficient

Previously, pick_ipc_buffer() compared two integers, which made the following true:

0 > 0x80000000

That would trigger assert in crm_ipc_prepare():

CRM_ASSERT(result != NULL);

Besides, crm_ipc_prepare() and pick_ipc_buffer() could suggest negative
values for PCMK_ipc_buffer.

This commit fixes these by using "unsigned int".