Get rid of some clang warnings.

Namely:

 * jpegoptim.c:333:13: warning: if statement has empty body [-Wempty-body]
    if (rcsid); /* so compiler won't complain about "unused" rcsid string */

 * jpegoptim.c:485:22: warning: the value of the size argument in 'strncat' is too large, might lead to a buffer overflow [-Wstrncat-size]
           strncat(tmpdir,"/",sizeof(tmpdir)-strlen(tmpdir));