Fix uninitialized read in MGAEnableSecondOutPut

`MGAGRestore` allocated `ModeInfo` without initializing its memory and then
passes it into `MGAEnableSecondOutPut` via `MGAEnableSecondOutPut(pScrn, &ModeInfo);`.

`MGAEnableSecondOutPut` then reads the unitialized memory in the line
`if ( !(pModeInfo->flSignalMode & POS_HSYNC) )`.

This patch just zeroes the memory to prevent those branches to be randomly
taken.

Signed-off-by: Raphael Isemann <teemperor@gmail.com>
6 jobs for FixUninitRead in 1 minute and 52 seconds (queued for 1 second)