MDEV-12144 Signal 6 crash corrupts ibd files

Avoid using STDERR_FILENO. The server uses freopen(stderr),
so stderr can be on any file descriptor.

include/my_global.h

@@ -421,9 +421,8 @@ extern "C" int madvise(void *addr, size_t len, int behav);
 #define SIGNAL_HANDLER_RESET_ON_DELIVERY
 #endif
 
-#ifndef STDERR_FILENO
-#define STDERR_FILENO fileno(stderr)
-#endif
+/* don't assume that STDERR_FILENO is 2, mysqld can freopen */
+#undef STDERR_FILENO
 
 /*
   Deprecated workaround for false-positive uninitialized variables

mysys/my_fopen.c

@@ -143,7 +143,7 @@ static int no_close(void *cookie __attribute__((unused)))
 /*
   A hack around a race condition in the implementation of freopen.
 
-  The race condition steams from the fact that the current fd of
+  The race condition stems from the fact that the current fd of
   the stream is closed before its number is used to duplicate the
   new file descriptor. This defeats the desired atomicity of the
   close and duplicate of dup2().

mysys/stacktrace.c

@@ -744,7 +744,7 @@ void my_safe_print_str(const char *val, int len)
 
 size_t my_write_stderr(const void *buf, size_t count)
 {
-  return (size_t) write(STDERR_FILENO, buf, count);
+  return (size_t) write(fileno(stderr), buf, count);
 }