Update air724.c

if (!str) 
    {
        return -1;
    }

devices/air724/air724.c

@@ -61,7 +61,6 @@ static int air724_signal_quality_check(void)
     char echo_buffer[32], *str;
     int try = 0;
 	
-	
     while (try++ < 10) 
     {
         tos_at_echo_create(&echo, echo_buffer, sizeof(echo_buffer), NULL);
@@ -72,6 +71,10 @@ static int air724_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;
@@ -97,6 +100,10 @@ static int air724_gsm_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CREG:%d,%d", &n, &stat);
         if (stat == 1)
         {
@@ -123,6 +130,10 @@ static int air724_gprs_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CGREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CGREG:%d,%d", &n, &stat);
         if (stat == 1)
         {
@@ -464,6 +475,10 @@ static int air724_parse_domain(const char *host_name, char *host_ip, size_t host
     */
     
     str = strstr(echo.buffer, ":");
+    if (!str) 
+    {
+        return -1;
+    }
     str += 1;
     if(*str == '0')
     {