common/io_exerciser: fix buffer overread in DataGenerator

common/io_exerciser: fix buffer overread in DataGenerator

Fix GCC-15 warning about reading uninitialized memory when copying
random data to fill remaining bytes in generated blocks.

The issue occurred when remainingBytes exceeded the 8-byte size of
the uint64_t rand1 variable, causing memcpy to read beyond the
variable's boundary. While this didn't cause crashes (reading from
stack) and the buffer was still properly filled with rand2, it
violated memory safety and generated compiler warnings.

Fixed by limiting the copy size to the actual size of the source
variable (sizeof(rand1)) to ensure we only read initialized memory.

Resolves GCC-15 warnings:

- DataGenerator.cc:76: memcpy reading 9-15 bytes from 8-byte region
- DataGenerator.cc:108: memcpy reading 9-15 bytes from 8-byte region

Signed-off-by: Kefu Chai <tchaikov@gmail.com>