Update m26.c

    if (!str) 
    {
        return -1;
    }

devices/m26/m26.c

@@ -55,7 +55,6 @@ static int m26_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);
@@ -66,6 +65,10 @@ static int m26_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;
@@ -91,6 +94,10 @@ static int m26_gsm_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CREG:%d,%d", &n, &stat);
         if (stat == 1)
         {
@@ -117,6 +124,10 @@ static int m26_gprs_network_check(void)
         }
 
         str = strstr(echo.buffer, "+CGREG:");
+        if (!str) 
+        {
+            return -1;
+        }
         sscanf(str, "+CGREG:%d,%d", &n, &stat);
         if (stat == 1)
         {
@@ -428,6 +439,10 @@ static int m26_parse_domain(const char *host_name, char *host_ip, size_t host_ip
 
     int seg1, seg2, seg3, seg4;
     str = strstr(echo.buffer, "OK");
+    if (!str) 
+    {
+        return -1;
+    }
     str += strlen("OK\r\n");
     sscanf(str, "%d.%d.%d.%d", &seg1, &seg2, &seg3, &seg4);
     snprintf(host_ip, host_ip_len, "%d.%d.%d.%d", seg1, seg2, seg3, seg4);