Fix conversion from 'UINTN' to 'UINT8' warnings

* MSVC generates two of the following in rtstr.c:
  warning C4244: 'function': conversion from 'UINTN' to 'UINT8', possible loss of data

lib/runtime/rtstr.c

@@ -71,7 +71,7 @@ RtStrnCpy (
 {
     UINTN Size = RtStrnLen(Src, Len);
     if (Size != Len)
-	RtSetMem(Dest + Len, '\0', (Len - Size) * sizeof(CHAR16));
+    RtSetMem(Dest + Len, '\0', (UINT8)((Len - Size) * sizeof(CHAR16)));
     RtCopyMem(Dest, Src, Size * sizeof(CHAR16));
 }
 
@@ -107,7 +107,7 @@ RtStpnCpy (
 {
     UINTN Size = RtStrnLen(Src, Len);
     if (Size != Len)
-	RtSetMem(Dest + Len, '\0', (Len - Size) * sizeof(CHAR16));
+    RtSetMem(Dest + Len, '\0', (UINT8)((Len - Size) * sizeof(CHAR16)));
     RtCopyMem(Dest, Src, Size * sizeof(CHAR16));
     return Dest + Size;
 }