libc/environ: add interrupt context check to getenv

Prevent getenv() from being called in interrupt context by adding
up_interrupt_context() check. This ensures thread-safe access to
environment variables in multi-threaded scenarios.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

sched/environ/env_getenv.c

@@ -67,7 +67,7 @@ FAR char *getenv(FAR const char *name)
 
   /* Verify that a string was passed */
 
-  if (name == NULL)
+  if (name == NULL || up_interrupt_context())
     {
       ret = -EINVAL;
       goto errout;