fix buffer overflow with huge number of subst args

fix buffer overflow with huge number of subst args

thanks to ole tange for reporting the issue.

echo a | ./jobflow.out -exec echo {} {} {} {} {} {} {} {} {} {} {} {} {} {} {} {} {} {} {}
=================================================================
==5173==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffd67da6970 at pc 0x7fb7c72b3904 bp 0x7ffd67d96740 sp 0x7ffd67d95ee8
WRITE of size 1 at 0x7ffd67da6970 thread T0
    #0 0x7fb7c72b3903 in __asan_memcpy (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x8c903)
    #1 0x4070e1 in substitute_all /home/rofl/jobflow/jobflow.c:544
    #2 0x407b1b in dispatch_line /home/rofl/jobflow/jobflow.c:635
    #3 0x408704 in main /home/rofl/jobflow/jobflow.c:752
    #4 0x7fb7c617082f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #5 0x401f18 in _start (/home/rofl/jobflow/jobflow.out+0x401f18)

Address 0x7ffd67da6970 is located in stack of thread T0 at offset 65888 in frame
    #0 0x407658 in dispatch_line /home/rofl/jobflow/jobflow.c:588

  This frame has 6 object(s):
    [32, 48) 'line_b'
    [96, 112) 'source_storage'
    [160, 176) '<unknown>'
    [224, 240) 'tilLastDot'
    [288, 304) '<unknown>'
    [352, 65888) 'subst_buf' <== Memory access at offset 65888 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow ??:0 __asan_memcpy