run-command.c: squelch a "use before assignment" warning

i686-apple-darwin9-gcc-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5490) compiler
(and probably others) mistakenly thinks variable failed_errno is used
before assigned.  Work it around by giving it a fake initialization.

Signed-off-by: David Soria Parra <dsp@php.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -19,7 +19,7 @@ int start_command(struct child_process *cmd)
 {
 	int need_in, need_out, need_err;
 	int fdin[2], fdout[2], fderr[2];
-	int failed_errno;
+	int failed_errno = failed_errno;
 
 	/*
 	 * In case of errors we must keep the promise to close FDs