Fix deletion of uninitialized POSIX TLS key

Zero (the initial value of context) is a valid TLS key, so doing this
can delete someone else's key.

Fixes #627.

src/posix_tls.h

@@ -37,6 +37,7 @@
 //
 typedef struct _GLFWtlsPOSIX
 {
+    GLFWbool        allocated;
     pthread_key_t   context;
 
 } _GLFWtlsPOSIX;