Change _stdio_handles_locking into _caller_handles_locking.

It is reported by tsan that funlockfile() can unlock an unlocked mutex.
It happens when printf() is called before fopen() or other stdio stuff.
As FLOCKFILE(fp) is called before __sinit(), _stdio_handles_locking is false,
and _FLOCK(fp) will not be locked. But then cantwrite(fp) in __vfprintf()
calls__sinit(), which makes _stdio_handles_locking become true, and
FUNLOCKFILE(fp) unlocks _FLOCK(fp).

Change _stdio_handles_locking into _caller_handles_locking,
so __sinit() won't change its value. Add test due to my previous fault.

Bug: 25392375
Change-Id: I483e3c3cdb28da65e62f1fd9615bf58c5403b4dd
diff --git a/libc/bionic/flockfile.cpp b/libc/bionic/flockfile.cpp
index b73907cb..db68801 100644
--- a/libc/bionic/flockfile.cpp
+++ b/libc/bionic/flockfile.cpp
@@ -40,7 +40,7 @@
     __sinit();
   }
 
-  if (fp != NULL) {
+  if (fp != nullptr) {
     pthread_mutex_lock(&_FLOCK(fp));
   }
 }
@@ -52,7 +52,7 @@
 
   // The specification for ftrylockfile() says it returns 0 on success,
   // or non-zero on error. So return an errno code directly on error.
-  if (fp == NULL) {
+  if (fp == nullptr) {
     return EINVAL;
   }
 
@@ -64,7 +64,7 @@
     __sinit();
   }
 
-  if (fp != NULL) {
+  if (fp != nullptr) {
     pthread_mutex_unlock(&_FLOCK(fp));
   }
 }