pipebuffer: More robust face null pointers.

It is really the caller responsibility not to call pipebuffer with null
buffers, etc. But don't let the crash happen here, and still asserting
early.

src/gallium/auxiliary/pipebuffer/pb_buffer.h

@@ -146,6 +146,8 @@ pb_map(struct pb_buffer *buf,
        unsigned flags)
 {
    assert(buf);
+   if(!buf)
+      return NULL;
    return buf->vtbl->map(buf, flags);
 }
 
@@ -154,6 +156,8 @@ static INLINE void
 pb_unmap(struct pb_buffer *buf)
 {
    assert(buf);
+   if(!buf)
+      return;
    buf->vtbl->unmap(buf);
 }
 
@@ -163,6 +167,12 @@ pb_get_base_buffer( struct pb_buffer *buf,
 		    struct pb_buffer **base_buf,
 		    unsigned *offset )
 {
+   assert(buf);
+   if(!buf) {
+      base_buf = NULL;
+      offset = 0;
+      return;
+   }
    buf->vtbl->get_base_buffer(buf, base_buf, offset);
 }
 
@@ -171,7 +181,8 @@ static INLINE void
 pb_destroy(struct pb_buffer *buf)
 {
    assert(buf);
-   assert(buf->vtbl);
+   if(!buf)
+      return;
    buf->vtbl->destroy(buf);
 }