intreadwrite.h: fix AV_RL32/AV_RB32 signedness.

The output type of the AV_RL32/AV_RB32 macros was signed int. The
resulting overflow broke at least some ASF streams with large
timestamps. Fix by adding a cast to uint32_t.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/intreadwrite.h

@@ -230,7 +230,7 @@ union unaligned_16 { uint16_t l; } __attribute__((packed)) av_alias;
 
 #ifndef AV_RB32
 #   define AV_RB32(x)                                \
-    ((((const uint8_t*)(x))[0] << 24) |         \
+    (((uint32_t)((const uint8_t*)(x))[0] << 24) |    \
                (((const uint8_t*)(x))[1] << 16) |    \
                (((const uint8_t*)(x))[2] <<  8) |    \
                 ((const uint8_t*)(x))[3])
@@ -246,7 +246,7 @@ union unaligned_16 { uint16_t l; } __attribute__((packed)) av_alias;
 
 #ifndef AV_RL32
 #   define AV_RL32(x)                                \
-    ((((const uint8_t*)(x))[3] << 24) |         \
+    (((uint32_t)((const uint8_t*)(x))[3] << 24) |    \
                (((const uint8_t*)(x))[2] << 16) |    \
                (((const uint8_t*)(x))[1] <<  8) |    \
                 ((const uint8_t*)(x))[0])