Update ec20.c

if (!str) 
    {
        return -1;
    }

devices/ec20/ec20.c

@@ -66,7 +66,6 @@ static int ec20_sim_card_check(void)
             return 0;
         }
     }
-		 
     return -1;
 }
 
@@ -87,6 +86,10 @@ static int ec20_signal_quality_check(void)
         }
 
         str = strstr(echo.buffer, "+CSQ:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CSQ:%d,%d", &rssi, &ber);
         if (rssi != 99) {
             return 0;
@@ -111,6 +114,10 @@ static int ec20_gsm_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CREG:%d,%d", &n, &stat);
         if (stat == 1) {
             return 0;
@@ -137,6 +144,10 @@ static int ec20_gprs_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CGREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CGREG:%d,%d", &n, &stat);
         if (stat == 1)
         {
@@ -172,7 +183,6 @@ static int ec20_set_apn(void)
         return -1;
     }
 
-
     tos_at_cmd_exec_until(&echo, 3000, "AT+QIACT=1\r\n");
     if (echo.status != AT_ECHO_STATUS_OK)
     {