check return value of fwrite() to fix compilation (with optimization enabled)
check return value of fwrite() to fix compilation (with optimization enabled)

this prevents
"
sql/src/backends/monet5/vaults/mseed.mx: In function ?MseedInternal?:
sql/src/backends/monet5/vaults/mseed.mx:295: error: ignoring return value of ?fwrite?, declared with attribute warn_unused_result
sql/src/backends/monet5/vaults/mseed.mx:296: error: ignoring return value of ?fwrite?, declared with attribute warn_unused_result
"

TODO:
Check whether more clean-up is required before bailing out in case fwrite() fails.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb