fix: stop segfaulting on non-existent directories

src/battery.c

@@ -3,6 +3,7 @@
 #include <stdbool.h>
 #include <unistd.h>
 
+#include "log.h"
 #include "utils.h"
 
 #include "config.h"
@@ -17,6 +18,10 @@ char
   /* open up the battery capacity for reading */
   d = concat(battery_status_path, "/capacity");
   f = fopen(d, "r");
+  if (!f) {
+    log_fatal("battery directory does not exist");
+    return NULL;
+  }
   free(d);
 
   /* create enough space for the battery percentage */