In main check for addressbok file open errors.

src/main.c

@@ -4,6 +4,7 @@
 #include "net.h"
 #include <stdio.h>
 #include <string.h>
+#include <errno.h>
 
 #define CHANNELS 1
 #define SAMPLE_RATE 48000
@@ -24,6 +25,10 @@ int main(int argc, char **argv) {
     addrbook_t ab = {0};
 
     FILE *in = fopen("addressbook_key", "r");
+    if (in == NULL) {
+        fprintf(stderr, "An addressbook file wasn't open: %s\n", strerror(errno));
+        return 1;
+    }
 
     addrbook_load(&ab, in);