PCX: Avoid segmentation fault with invalid file.

If a PCX file contains a bytesperline entry which is too small, it is
possible to trigger an out of boundary read, which can lead to a
segmentation fault.

The bytesperline validation is incomplete. While checking if enough
bytes per line exist, the integer truncation during the division must be
taken into account.

An example would be a 1x1 PCX file with a bpp of 1 (monochrome). The
current check allows a bytesperline field of 0, which in turn would lead
to a 0 byte allocation in load_1. Yet, the code would access index 0.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

plug-ins/common/file-pcx.c

@@ -409,7 +409,7 @@ load_image (const gchar  *filename,
       fclose (fd);
       return -1;
     }
-  if (bytesperline < (width * pcx_header.bpp) / 8)
+  if (bytesperline < ((width * pcx_header.bpp + 7) / 8))
     {
       g_message (_("Invalid number of bytes per line in PCX header"));
       fclose (fd);