tjexample.c: Fix error when recompressing

(regression introduced by fc01f4673b)

Because of the TurboJPEG 3 API overhaul, the pure compression code path
in tjexample.c now creates a TurboJPEG compression instance prior to
calling tj3LoadImage().  However, due to an oversight, a compression
instance was no longer created in the recompression code path.  Thus,
that code path attempted to reuse the TurboJPEG decompression instance,
which caused an error ("tj3Set(): TJPARAM_QUALITY is not applicable to
decompression instances.")  This commit modifies tjexample.c so that the
recompression code path creates a new TurboJPEG compression instance if
one has not already been created.

Fixes #716

tjexample.c

@@ -350,6 +350,7 @@ int main(int argc, char **argv)
                        pixelFormat) < 0)
       THROW_TJ("decompressing JPEG image");
     tj3Free(jpegBuf);  jpegBuf = NULL;
+    tj3Destroy(tjInstance);  tjInstance = NULL;
   } else {
     /* Input image is not a JPEG image.  Load it into memory. */
     if ((tjInstance = tj3Init(TJINIT_COMPRESS)) == NULL)
@@ -379,6 +380,8 @@ int main(int argc, char **argv)
     printf(", %s subsampling, quality = %d\n", subsampName[outSubsamp],
            outQual);
 
+    if (!tjInstance && (tjInstance = tj3Init(TJINIT_COMPRESS)) == NULL)
+      THROW_TJ("initializing compressor");
     if (tj3Set(tjInstance, TJPARAM_SUBSAMP, outSubsamp) < 0)
       THROW_TJ("setting TJPARAM_SUBSAMP");
     if (tj3Set(tjInstance, TJPARAM_QUALITY, outQual) < 0)