crash_handler: write backtrace to stderr instead of backtrace file

the backtrace file is hard to collect in some environment but the stdout
and stderr is usually captured by default.

lib/src/common/crash_handler.cc

@@ -28,16 +28,12 @@ void srsran_debug_handle_crash(int argc, char** argv)
 #include "srsran/common/backtrace.h"
 #include "srsran/version.h"
 
-const static char crash_file_name[] = "./srsRAN.backtrace.crash";
 static int    bt_argc;
 static char** bt_argv;
 
 static void crash_handler(int sig)
 {
-  FILE* f = fopen(crash_file_name, "a");
-  if (!f) {
-    printf("srsRAN crashed... we could not save backtrace in '%s'...\n", crash_file_name);
-  } else {
+  FILE*     f = stderr;
   time_t    lnTime;
   struct tm stTime;
   char      strdate[32];
@@ -54,12 +50,10 @@ static void crash_handler(int sig)
   fprintf(f, "' version=%s signal=%d date='%s' ---\n", SRSRAN_VERSION_STRING, sig, strdate);
 
   srsran_backtrace_print(f);
-    fprintf(f, "\n");
 
-    printf("srsRAN crashed... backtrace saved in '%s'...\n", crash_file_name);
-    fclose(f);
-  }
-  printf("---  exiting  ---\n");
+  fprintf(f, "srsRAN crashed. Please send this backtrace to the developers ...\n");
+
+  fprintf(f, "---  exiting  ---\n");
   exit(1);
 }