Fix compile error with FORTIFY_SOURCE

You're gonna have a bad day if you attempt to read 3 bytes
into 1 byte.

(cherry picked from commit b4f59474493a0793375e723edbd0d29e55a8c581)

Change-Id: I6beea5631f30f4aa04ebbed10207f399187bf218
diff --git a/tools/kfmapp/kfmapp.c b/tools/kfmapp/kfmapp.c
index d35a64f..1f2b923 100644
--- a/tools/kfmapp/kfmapp.c
+++ b/tools/kfmapp/kfmapp.c
@@ -731,7 +731,7 @@
 
 int fmapp_get_rx_rssi_threshold(void)
 {
-    unsigned char fm_rssi_threshhold;
+    unsigned char fm_rssi_threshhold[4];
     int fd, res;
 
     fd = open(FMRX_RSSI_LVL_SYSFS_ENTRY, O_RDONLY);
@@ -746,6 +746,7 @@
                 FMRX_RSSI_LVL_SYSFS_ENTRY,strerror(res));
         goto exit;
     }
+    fm_rssi_threshhold[3] = '\0';
 
     printf("Current FM RSSI threshold level is %d \n",
             atoi((char *) &fm_rssi_threshhold));