More header checks (CCBUG: 479612)
Added header checks to limit false positives. See also CCBUG: 479612.
From official TGA docs:
Color Map Specification - Field 4 (5 bytes):
This field and its sub-fields describe the color map (if any) used for the image. If the Color Map Type field
is set to zero, indicating that no color map exists, then these 5 bytes should be set to zero. These bytes
always must be written to the file.
Edited by Mirco Miranda