lavfi/vulkan: allow calling glslang_uninit without a prior init

Allows us to uninit cleanly.
This assert was also somewhat pointless as we assert every other
function, so another assert would be triggered long before this
one is.

libavfilter/glslang.cpp

@@ -235,8 +235,7 @@ int glslang_init(void)
 void glslang_uninit(void)
 {
     pthread_mutex_lock(&glslang_mutex);
-    av_assert0(glslang_refcount > 0);
-    if (--glslang_refcount == 0)
+    if (glslang_refcount && (--glslang_refcount == 0))
         FinalizeProcess();
     pthread_mutex_unlock(&glslang_mutex);
 }