During DXE core memory service initialization, the system would check
available resource descriptor hobs against the memory top from PHIT hob.
However, it is possible that a given resource descriptor hob will not be
larger than the cover the memory top, causing the Length calculation to
underflow.
This change adds a check for potential underflow before performing the
subtraction.
Signed-off-by: Kun Qin <kun.qin@microsoft.com>
feeb137d43