HTTP: fix stack overwrite due to format string error.

HTTP: fix stack overwrite due to format string error.

On AMD64, %lu refers to a 64-bit unsigned value, but the address passed
to sscanf points to a 32-bit unsigned value.  This causes an adjoining
value on the stack to be overwritten with data from the converted
integer.  Fix the format string to match the size of the supplied value,
and remove the pointer cast.