This patch fixes the handling of the corner case where the allocation of
a trace buffer throws 'Out_of_caps' or 'Out_of_ram'. Under this
circumstance, the '_buffer' would still be flagged with the 'size',
which prevented any subsequent allocation attempt. This patch fixes the
problem by initializing the 'size' after the potentially throwing
allocation.
The problem triggered with the test-trace_logger after the accounting of
core's TRACE service (replacing the 'Allocator_guard' by
'Constrained_ram_allocator') became more accurate.
Related to issue #3750
64bc008c3a