libavutil/softfloat: Fix av_normalize1_sf bias.

av_normalize1_sf doesn't properly address border case when mantis is
exactly -0x40000000.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 7131aba9)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/softfloat.h

@@ -59,7 +59,7 @@ static av_const SoftFloat av_normalize_sf(SoftFloat a){
 
 static inline av_const SoftFloat av_normalize1_sf(SoftFloat a){
 #if 1
-    if((int32_t)(a.mant + 0x40000000U) < 0){
+    if((int32_t)(a.mant + 0x40000000U) <= 0){
         a.exp++;
         a.mant>>=1;
     }