[bug-gawk] Assert error in `debug.c:set_breakpoint` triggered by an empty `b` command

[Ray Song]

gawk-4.1.0
debug.c:2318          assert(rp != NULL);

Here is a 

   % ./gawk --version
   GNU Awk 4.1.0, API: 1.0 (GNU MPFR 3.1.1-p2, GNU MP 5.1.0)
   ...
   
   % ./gawk -D -f =(echo 1)
   gawk> b 1
   Breakpoint 1 set at file `/tmp/zshTH8H8Y', line 1
   gawk> r
   Starting program:
   a
   Stopping in Rule ...
   Breakpoint 1, main() at `/tmp/zshTH8H8Y':1
   1       1
   gawk> cle
   Deleted breakpoint 1
   gawk> b
   gawk: /tmp/zshTH8H8Y:1: (FILENAME=- FNR=1) fatal error: internal error: 
segfault
   [1]    767 abort      ./gawk -D -f =(echo 1)