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>

src/mga_dacG.c

@@ -1754,6 +1754,7 @@ MGA_NOT_HAL(
 	} else {
 	   /* Second Crtc */
 	   xMODEINFO ModeInfo;
+	   memset( &ModeInfo, 0, sizeof(ModeInfo) );
 
 MGA_NOT_HAL(
 	   /* Enable Dual Head */