nptl threads: Fix bug in using a weak variable.

In case __nptl_nthreads is not defined in the final binary then avoid
dereferencing the address since it's null. Anyway this means there is
no other thread and so we must exit.

	* libc/misc/internals/__uClibc_main.c (__uClibc_main): Handle case
	where &__nptl_nthreads is null.

Signed-off-by: Mickaël Guêné <mickael.guene@st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@st.com>

libc/misc/internals/__uClibc_main.c

@@ -544,7 +544,7 @@ void __uClibc_main(int (*main)(int, char **, char **), int argc,
 		result = 0;
 		unsigned int *const ptr = &__nptl_nthreads;
 
-		if (! atomic_decrement_and_test (ptr))
+		if (ptr && ! atomic_decrement_and_test (ptr))
 			/* Not much left to do but to exit the thread, not the process.  */
 			__exit_thread_inline (0);
 	}