Fix checks for reading and writing FuseMessage.

Previously FuseMessage were checking result of read/write operation
after checking header.len value is valid. This was wrong because
header.len does not contain correct value when read function does not
read any bytes and returns zero.

Bug: 33278098
Test: libappfuse_test
Change-Id: Icf998ca6c3eeee20cbc4aa2f65195a87e59ffc27