Fix fatal compiler error that occurred when an %ffi extension point contained invalid JavaScript

[mediremi]

When editing invalid JS in an %ffi, my compiler would sometimes fail with:

FAILED: MyFile.ast

  Warning number 103
  /home/mediremi/MyFile.res:30:9-10
Fatal error: exception Invalid_argument("String.sub / Bytes.sub")

Enabling backtraces with Printexc.record_backtrace true gave me:

Fatal error: exception Invalid_argument("String.sub / Bytes.sub")
Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
Called from Stdlib__String.sub in file "string.ml" (inlined), line 43, characters 2-23
Called from Code_frame.print in file "compiler/ml/code_frame.ml", lines 139-140, characters 4-57
Called from Location.print in file "compiler/ml/location.ml", lines 143-144, characters 8-57
...

This PR prevents this fatal error by guarding String.sub with a check for its pos and len arguments (in this case, start_line_line_offset and end_line_line_end_offset - start_line_line_offset).

With this fix, the compiler now successfully reports the warning triggered by my invalid JS:

rescript: [1/6] MyFile.ast

  Warning number 103
  /home/mediremi/MyFile.res:30:9-10


  FFI warning: Unexpected reserved word

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commit: 52b065d