deadlock in waitchld()

[Roman Rakus]

Bash hangs in wait4() (WAITPID) if TERM signal trap handler is executed 
during execution of pipeline.

RR
Reproducer:
#!/bin/bash 

trap "/bin/echo trapped $$" TERM 

printf '%d\n' $$ 

while :; do 

  dd if=/dev/zero bs=1k count=128 2>&1 | cat > /dev/null 

done

and bombard the bash process with TERM signals:
#!/bin/bash
while :; do
  kill -TERM $1 || break
  usleep 100000
done