pathtrace_select() is never called with NULL, remove dead code

pathtrace_select() is never called with NULL, remove dead code

pathtrace_select() is only called for -P FILE options,
and FILE is never a NULL pointer.

   text	   data	    bss	    dec	    hex	filename
 239453	    672	  19012	 259137	  3f441	strace.before
 239329	    672	  19012	 259013	  3f3c5	strace

* pathtrace.c (pathtrace_select): Remove "if (path == NULL)...".
(pathtrace_select): Remove code which only executes if path == NULL.
The code was also buggy, it can free non-malloced pointer.
(getfdpath): Simplify snprintf to sprintf.
(pathmatch): Use strcmp() == 0 idiom for string equality test.
(pathtrace_match): Likewise.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>