ATHandler::read_int: allow returning 0 successfully

features/cellular/framework/AT/ATHandler.cpp

@@ -722,7 +722,22 @@ int32_t ATHandler::read_int()
         return -1;
     }
 
-    return std::strtol(buff, NULL, 10);
+    errno = 0;
+    char *endptr;
+    long result = std::strtol(buff, &endptr, 10);
+    if ((result == LONG_MIN || result == LONG_MAX) && errno == ERANGE) {
+        return -1; // overflow/underflow
+    }
+    if (result < 0) {
+        return -1; // negative values are unsupported
+    }
+    if (*buff == '\0') {
+        return -1; // empty string
+    }
+    if (*endptr != '\0') {
+        return -1; // trailing garbage
+    }
+    return (int32_t) result;
 }
 
 void ATHandler::set_delimiter(char delimiter)

features/cellular/framework/AT/ATHandler.h

@@ -410,9 +410,9 @@ public:
      */
     ssize_t read_hex_string(char *str, size_t size);
 
-    /** Reads as string and converts result to integer. Supports only positive integers.
+    /** Reads as string and converts result to integer. Supports only non-negative integers.
      *
-     *  @return the positive integer or -1 in case of error.
+     *  @return the non-negative integer or -1 in case of error.
      */
     int32_t read_int();