ext2fs: fix a check for negative block numbers.
ext2fs: fix a check for negative block numbers.

The previous change accidentally left the substraction we
were trying to avoid in case that i_blocks could become
negative.

Reported by:	bde
MFC after:	4 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase