oem-ibm: avoid file read range integer overflow

Fix IBM OEM file-read range validation to avoid overflowing the
32-bit offset + length boundary check.

A PLDM peer could issue ReadFile with a valid small offset and a
large length, such as offset=1 and length=UINT32_MAX. The previous
check computed offset + length directly, allowing the sum to wrap and
skip the clamp logic. In the generic read path this could trigger an
attempt to allocate a response near 4 GiB, causing std::bad_alloc and
terminating pldmd.

Use the overflow-safe form length > fileSize - offset after the
existing offset >= fileSize validation. This preserves the existing
clamp-at-EOF behavior while fixing all affected IBM file-read paths,
including generic, DMA, and file-by-type reads.

Change-Id: I774e0a16b9bf855b73b1456cec9a00e93fcc4cad
Signed-off-by: Manjeet Singh <itsmanjeet1998@gmail.com>