Don't keep trying to decompress the same image if tj3Decompress*() has
already thrown an error. Otherwise, if the image has an excessive
number of scans, then each iteration of the loop will try to decompress
up to the scan limit, which may cause the overall test to time out even
if one iteration doesn't time out.
The non-default options were not being tested because of a pixel format
comparison buglet. This commit also changes the code in both
decompression fuzz targets such that non-default options are tested
based on the pixel format index rather than the pixel format value,
which is a bit more idiot-proof.
Otherwise, the targets will require libstdc++, the i386 version of which
is not available in the OSS-Fuzz runtime environment. The OSS-Fuzz
build environment passes -stdlib:libc++ in the CXXFLAGS environment
variable in order to mitigate this issue, since the runtime environment
has the i386 version of libc++, but using that compiler flag requires
using the C++ compiler.
