Bug 26216

On pthread environment, each thread has their own stack limit.
when user requested larger than current thread's stack size to alloca(),
alloca() return invalid pointer.
It must be NULL pointer.

How-To-Repeat: gcc -ansi -pedantic -g -Wall -pthread -o test test.c
./test
./test <stacksize>

--
/* test.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <pthread.h>

/* the stderr time ticker thread */
static void *ticker(void *_arg)
{
	int		kb = (int)_arg;
    char	*ptr;

	if (NULL != (ptr = alloca(kb * 1024))) {
		printf("alloca(%dKB) return %p\n", kb, ptr);
		memset(ptr, 0xdf, kb * 1024);
	}
	else {
		printf("alloca(%dKB) return NULL\n", kb);
	}

	return NULL;
}

int main(int argc, char *argv[])
{
	pthread_attr_t	thread_attr;
	pthread_t		ticker_thread;
	size_t			stacksize;
    int				i;

    signal(SIGPIPE, SIG_IGN);

	pthread_attr_init(&thread_attr);
	pthread_attr_setdetachstate(&thread_attr, PTHREAD_CREATE_DETACHED);
	if (2 == argc) {
		stacksize = (size_t) strtoul(argv[1], (char **)NULL, 10);
		if (!pthread_attr_setstacksize(&thread_attr, stacksize*1024)) {
			printf("set stacksize = %dKB\n", stacksize);
		}
	}
	if (!pthread_attr_getstacksize(&thread_attr, &stacksize)) {
		printf("stack size = %dKB\n", stacksize/1024);
	}

    for (i = 4; i < 65536 ; i += 8) {
		pthread_create(&ticker_thread, &thread_attr, ticker, (void *)i);
		sleep(1);
    }

	return 0;
}