gossipstore: fix uninitialized input fd

Initialize infd to STDIN_FILENO if the input file argument is missing.

Caught with gcc version: 7.4.0

devtools/create-gossipstore.c: In function ‘main’:
devtools/create-gossipstore.c:130:9: error: ‘infd’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
  while (read_all(infd, &be_inlen, sizeof(be_inlen))) {

Suggested-by: @ZmnSCPxj <https://github.com/ElementsProject/lightning/pull/2674#issuecomment-495617253>
Signed-off-by: William Casarin <jb55@jb55.com>

devtools/create-gossipstore.c

@@ -115,6 +115,8 @@ int main(int argc, char *argv[])
 		if (infd < 0)
 			err(1, "opening %s", infile);
 	}
+	else
+		infd = STDIN_FILENO;
 
 	if (outfile) {
 		outfd = open(outfile, O_WRONLY|O_TRUNC|O_CREAT, 0666);