MRF: DeflateBlock(): avoid potential buffer overrun

[rouault]

Fixes Coverity 1432696

Relevant lines of Coverity analysis

827    {

CID 1432696: (#1 of 1): Out-of-bounds access (OVERRUN)
21. overrun-buffer-arg: Overrunning dynamic array filedst.buffer by passing it to a function that accesses it at byte filedst.size + 63.[show details]
828        usebuff = DeflateBlock(filedst, poMRFDS->pbsize - filedst.size,
829                               deflate_flags);

291    // If we allocated a buffer, we need to copy the data to the input buffer.
7. access_dbuff_in_call: Calling memcpy indexes array src.buffer with index src.size.
292    memcpy(src.buffer, dbuff, src.size);

CC @lucianpls
At the beginning I thought this was a false positive from Coverity Scan but reading and re-reading the code again, I believe it is right and at least theoritically we could potentially overrun src.buffer, hence the check I've added (I'm not sure Coverity will realize that this added checks prevents the overrun from happening)

[coveralls]

coverage: 70.702% (-0.02%) from 70.719%
when pulling 2c09d2c on rouault:mrf_deflateblock
into 19671ee on OSGeo:master.