vga: Don't switch to 1 x 1 character text screen

Initially, vga_get_text_resolution returns a text resolution of 1 x 1
(vga register values are 0).

This is visible during MIPS Malta boot with SDL. It also occurs with the
i386 or x86_64 system emulation when it runs in single step mode:

QEMU changes the size of the SDL window to the smallest possible value
which is supported by the window manager. As this is not the calculated
size, QEMU switches to scaled mode. When the BIOS or the VGA driver sets
the normal text resolution, the window stays small and displays
microscopic characters.

Ignoring text resolutions of 1 x 1 or less avoids these problems.
A similar workaround already exists for too large resolutions.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/vga.c

@@ -1327,6 +1327,10 @@ static void vga_draw_text(VGACommonState *s, int full_update)
     line_offset = s->line_offset;
 
     vga_get_text_resolution(s, &width, &height, &cw, &cheight);
+    if ((height * width) <= 1) {
+        /* better than nothing: exit if transient size is too small */
+        return;
+    }
     if ((height * width) > CH_ATTR_SIZE) {
         /* better than nothing: exit if transient size is too big */
         return;