make sure we always initialize variables imin and imax before we use them
make sure we always initialize variables imin and imax before we use them

and simplify the respective code a bit

this also fixes compilation with optimization enabled
by preventing
"
sql/src/backends/monet5/vaults/mseed.mx: In function ?MseedInternal?:
sql/src/backends/monet5/vaults/mseed.mx:205: error: ?imin? may be used uninitialized in this function
sql/src/backends/monet5/vaults/mseed.mx:205: error: ?imax? may be used uninitialized in this function
"

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