avutil/softfloat: Check for MIN_EXP in av_sqrt_sf()

Otherwise the exponent could eventually underflow
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/softfloat.h

@@ -181,6 +181,10 @@ static av_always_inline SoftFloat av_sqrt_sf(SoftFloat val)
             val.mant >>= 1;
 
         val.exp = (val.exp >> 1) + 1;
+        if (val.exp < MIN_EXP) {
+            val.exp = MIN_EXP;
+            val.mant= 0;
+        }
     }
 
     return val;