Bug report: tac crash

[Daniel Dunbar]

Hi,

The following crashes tac on my machine (6.10 and 6.11):
--
$ echo > x; tac -r x x
Segmentation fault
--

The crash occurs in re_copy_regs and the error has to do with the code 
assuming regs->num_regs is initialized when it is not, in conjunction with 
the regs_allocated parameter. This results in the final loop scribbling -1 over
varying and often large amounts of memory.

It looks to me like the problem is re_copy_regs assuming the state of the 
re_pattern_buffer and the re_registers are in sync, but this doesn't hold when
tac makes multiple re_search calls with the registers on the stack. 

 - Daniel