Set at least PTHREAD_STACK_MIN stack size when creating threads.

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 797557d76f830cde51cb37943edce6f5d6f3504b..923cd8700ed9a062c8ed8ad7a505da253ae084dd 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -564,6 +564,9 @@ Thread::Thread(const Options& options)
     : data_(new PlatformData),
       stack_size_(options.stack_size()),
       start_semaphore_(NULL) {
+  if (stack_size_ > 0 && stack_size_ < PTHREAD_STACK_MIN) {
+    stack_size_ = PTHREAD_STACK_MIN;
+  }
   set_name(options.name());
 }