zwave_api: Harden zwapi_connection.c by checking snprintf

Checking snprintf results, reminder :

    If the output was truncated due to this limit, then the return
    value is the number of characters (excluding the terminating
    null byte) which would have been written to the final string if
    enough space had been available

This was found using CodeQL:

    Potential fix for code scanning alert no. 19:
    Potentially overflowing call to snprintf

More refactoring can be done in later change

Origin: SiliconLabsSoftware#113
Relate-to: SiliconLabsSoftware#100
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>
Signed-off-by: Philippe Coval <philippe.coval@silabs.com>

Author: rzr

applications/zpc/components/zwave_api/src/zwapi_connection.c

@@ -11,6 +11,7 @@
  *
  *****************************************************************************/
 
+#include <assert.h>
 #include <string.h>
 #include "zwapi_connection.h"
 #include "zwapi_serial.h"
@@ -45,19 +46,44 @@ static const char *zwapi_frame_to_string(const uint8_t *buffer,
 {
   static char message[1000] = {'\0'};
   uint16_t index            = 0;
+  int written               = 0;
   for (uint16_t i = 0; i < buffer_length; i++) {
     if (i == 0) {
       // Don't log the SOF byte.
       continue;
     } else if (i == 1) {
-      index += snprintf(message + index, sizeof(message) - index, "Length=");
+      written = snprintf(message + index, sizeof(message) - index, "Length=");
+      if (written < 0 || written >= sizeof(message) - index) {
+        sl_log_error(LOG_TAG, "Overflow in zwapi_frame_to_string\n");
+        assert(false);
+        return NULL;
+      }
+      index += written;
     } else if (i == 2) {
-      index += snprintf(message + index, sizeof(message) - index, "Type=");
+      written = snprintf(message + index, sizeof(message) - index, "Type=");
+      if (written < 0 || written >= sizeof(message) - index) {
+        sl_log_error(LOG_TAG, "Overflow in zwapi_frame_to_string\n");
+        assert(false);
+        return NULL;
+      }
+      index += written;
     } else if (i == 3) {
-      index += snprintf(message + index, sizeof(message) - index, "Cmd=");
+      written = snprintf(message + index, sizeof(message) - index, "Cmd=");
+      if (written < 0 || written >= sizeof(message) - index) {
+        sl_log_error(LOG_TAG, "Overflow in zwapi_frame_to_string\n");
+        assert(false);
+        return NULL;
+      }
+      index += written;
+    }
+    written
+      = snprintf(message + index, sizeof(message) - index, "%02X ", buffer[i]);
+    if (written < 0 || written >= sizeof(message) - index) {
+      sl_log_error(LOG_TAG, "Overflow in zwapi_frame_to_string\n");
+      assert(false);
+      return NULL;
     }
-    index
-      += snprintf(message + index, sizeof(message) - index, "%02X ", buffer[i]);
+    index += written;
   }
   return message;
 }