run-command: avoid undefined behavior in exists_in_PATH

run-command: avoid undefined behavior in exists_in_PATH

In this function, we free the pointer we get from locate_in_PATH and
then check whether it's NULL.  However, this is undefined behavior if
the pointer is non-NULL, since the C standard no longer permits us to
use a valid pointer after freeing it.

The only case in which the C standard would permit this to be defined
behavior is if r were NULL, since it states that in such a case "no
action occurs" as a result of calling free.

It's easy to suggest that this is not likely to be a problem, but we
know that GCC does aggressively exploit the fact that undefined
behavior can never occur to optimize and rewrite code, even when that's
contrary to the expectations of the programmer.  It is, in fact, very
common for it to omit NULL pointer checks, just as we have here.

Since it's easy to fix, let's do so, and avoid a potential headache in
the future.

Noticed-by: Miriam R. <mirucam@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>