trap: fix use after free (#446)

This commit adds a fix for the trap command, backported from a fork
of ksh2020: 2033375f

src/cmd/ksh93/sh/jobs.c: job_chldtrap():
- Fixed a use after free bug in the for loop. The string pointed to
  by sh.st.trapcom[SIGCHLD] may be freed from memory after
  sh_trap(), so it must be reobtained each time sh_trap() is called
  from within the for loop.

NEWS

@@ -9,6 +9,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   values. Previously, arithmetic $((var=1)) or conditional ${var:=value}
   assignments were not exported even with allexport on.
 
+- Fixed a memory leak that could occur when using traps.
+
+- Fixed a use after free issue that could occur when modifying or setting
+  a SIGCHLD trap.
+
 2022-02-01:
 
 - Upon invocation, the interactive shell no longer leaves the user without