avutil: av_x_if_null: remove const for the return value.

This is the usual weirdness with const/non-const pointers through a function.
See the prototype of strchr for a standard example.

The function had to be moved down, below the point where stdint.h is
indirectly included.
Signed-off-by: Nicolas George <nicolas.george@normalesup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avutil.h

@@ -324,14 +324,6 @@ char av_get_picture_type_char(enum AVPictureType pict_type);
  * @}
  */
 
-/**
- * Return x default pointer in case p is NULL.
- */
-static inline const void *av_x_if_null(const void *p, const void *x)
-{
-    return p ? p : x;
-}
-
 #include "common.h"
 #include "error.h"
 #include "mathematics.h"
@@ -340,6 +332,14 @@ static inline const void *av_x_if_null(const void *p, const void *x)
 #include "log.h"
 #include "pixfmt.h"
 
+/**
+ * Return x default pointer in case p is NULL.
+ */
+static inline void *av_x_if_null(const void *p, const void *x)
+{
+    return (void *)(intptr_t)(p ? p : x);
+}
+
 /**
  * @}
  * @}