OTA: STM32H7 add check on fopen failures

Co-authored-by: Andrea Gilardoni <4046444+andreagilardoni@users.noreply.github.com>

src/ota/implementation/OTASTM32H7.cpp

@@ -62,6 +62,9 @@ OTACloudProcessInterface::State STM32H7OTACloudProcess::startOTA() {
 
   decompressed = fopen(_filename.c_str(), "wb");
 
+  if(decompressed == nullptr) {
+    return ErrorOpenUpdateFileFail;
+  }
   // start the download if the setup for ota storage is successful
   return OTADefaultCloudProcessInterface::startOTA();
 }