build: handle possible fscanf errors

on gcc (GCC) 7.4.0

devtools/create-gossipstore.c: In function ‘load_scid_file’:
devtools/create-gossipstore.c:22:9: error: ignoring return value of ‘fscanf’ ...
         fscanf(scidfd, "%d\n", &n);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
devtools/create-gossipstore.c:24:9: error: ignoring return value of ‘fscanf’ ...
         fscanf(scidfd, "%s\n", title);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [<builtin>: devtools/create-gossipstore.o] Error 1

Signed-off-by: William Casarin <jb55@jb55.com>

devtools/create-gossipstore.c

@@ -18,10 +18,14 @@
 
 struct scidsat * load_scid_file(FILE * scidfd)
 {
-        int n;
-        fscanf(scidfd, "%d\n", &n);
+        int n, ok;
+        ok = fscanf(scidfd, "%d\n", &n);
+	if (ok == EOF)
+		return NULL;
 	char title[16];
-        fscanf(scidfd, "%s\n", title);
+        ok = fscanf(scidfd, "%s\n", title);
+	if (ok == EOF)
+		return NULL;
 	struct scidsat * scids = calloc(n, sizeof(scidsat));
 	int i = 0;
         while(fscanf(scidfd, "%s ,%ld\n", scids[i].scid, &scids[i].sat.satoshis) == 2 ) { /* Raw: read from file */