vgabios: Avoid possible divide by zero in bochsvga_set_displaystart.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/bochsvga.c b/vgasrc/bochsvga.c
index a19eccb..6da9d5d 100644
--- a/vgasrc/bochsvga.c
+++ b/vgasrc/bochsvga.c
@@ -212,6 +212,8 @@
     if (GET_GLOBAL(dispi_found)) {
         int bpp = vga_bpp(vmode_g);
         int linelength = dispi_read(VBE_DISPI_INDEX_VIRT_WIDTH) * bpp / 8;
+        if (!linelength)
+            return 0;
         dispi_write(VBE_DISPI_INDEX_X_OFFSET, (val % linelength) * 8 / bpp);
         dispi_write(VBE_DISPI_INDEX_Y_OFFSET, val / linelength);
     }