Fix potential memory allocation problem
Fix potential memory allocation problem

If we allocate space for a substring, and that triggers a gc, the
original string won't be in the same place, so we need to account for
this in idris_substr.

kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev