BUG/MINOR: mworker-prog: Fix segmentation fault during cfgparse

BUG/MINOR: mworker-prog: Fix segmentation fault during cfgparse

Consider this configuration:

    frontend fe_http
    	mode http
    	bind *:8080

    	default_backend be_http

    backend be_http
    	mode http
    	server example example.com:80

    program foo bar

Running with valgrind results in:

    ==16252== Invalid read of size 8
    ==16252==    at 0x52AE3F: cfg_parse_program (mworker-prog.c:233)
    ==16252==    by 0x4823B3: readcfgfile (cfgparse.c:2180)
    ==16252==    by 0x47BCED: init (haproxy.c:1649)
    ==16252==    by 0x404E22: main (haproxy.c:2714)
    ==16252==  Address 0x48 is not stack'd, malloc'd or (recently) free'd

Check whether `ext_child` is valid before attempting to free it and its
contents.

This bug was introduced in 9a1ee7ac31c56fd7d881adf2ef4659f336e50c9f.
This fix must be backported to HAProxy 2.0.