burst_buffer/cray - fix script_argv use-after-free.

==8640== Thread 5 bckfl:
==8640== Syscall param openat(filename) points to unaddressable byte(s)
==8640==    at 0x4A81D0E: open (open64.c:48)
==8640==    by 0x5934ABB: _update_job_env (burst_buffer_cray.c:3338)
==8640==    by 0x5934ABB: bb_p_job_begin (burst_buffer_cray.c:3962)
...
==8640==  Address 0x6b96120 is 16 bytes inside a block of size 61 free'd
==8640==    at 0x48369AB: free (vg_replace_malloc.c:530)
==8640==    by 0x49D4873: slurm_xfree (xmalloc.c:244)
==8640==    by 0x490C317: free_command_argv (run_command.c:249)
==8640==    by 0x5934A5C: bb_p_job_begin (burst_buffer_cray.c:3947)
...
==8640==  Block was alloc'd at
==8640==    at 0x4837B65: calloc (vg_replace_malloc.c:752)
==8640==    by 0x49D4566: slurm_xmalloc (xmalloc.c:87)
==8640==    by 0x49D4B67: makespace (xstring.c:103)
==8640==    by 0x49D4C91: _xstrcat (xstring.c:134)
==8640==    by 0x49D4ECF: _xstrfmtcat (xstring.c:280)
==8640==    by 0x593497C: bb_p_job_begin (burst_buffer_cray.c:3936)
...

Bug 6807.