inline.h: Add comment for the cast that might appear redundant at first glance.

(intended to be squashed before merge)

Author: t-a-k

inline.h

@@ -3582,6 +3582,8 @@ S_uv_mul_overflow (UV auv, UV buv, UV *const result)
     /* else: eg 32 bit is at most 0xFFFF * 0xFFFF == 0xFFFE0001
        so the unsigned multiply cannot overflow.  */
 
+    /* (UV) cast below is necessary to force the multiplication to produce
+       UV result, as alow and blow might be narrower than UV */
     UV product_low = (UV)alow * blow;
     return S_uv_add_overflow(product_middle, product_low, result);
 }