im-sandbox: fix bounds of snprintf statements

I was noticing a segmentation fault appearing at the end of an indexamajig call. Debugging gave me:

snprintf (__fmt=0x40d1fd "%s/mosflm.lp", __n=127, __s=0x471f90 "")
delete_temporary_folder (n_proc=12, tmpdir=0x427c00 "./indexamajig.260408") at ../src/im-sandbox.c:1100

Which led me to these snprintf statements. Easily fixed, but a bit hard to spot.