fix compiling errors with clang-8

Compiling with clang-8 fails with the following errors:

largeNbDicts.c:562:37: error: implicit conversion turns floating-point
number into integer: 'const double' to 'U64' (aka 'unsigned long')
[-Werror,-Wfloat-conversion]
        U64 const dTime_ns = result.nanoSecPerRun;
                  ~~~~~~~~   ~~~~~~~^~~~~~~~~~~~~

zstdcli.c:300:5: error: '@return' command used in a comment that is
not attached to a function or method declaration
[-Werror,-Wdocumentation]
 * @return 1 means that cover parameters were correct
   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

zstdcli.c:301:5: error: '@return' command used in a comment that is
not attached to a function or method declaration
[-Werror,-Wdocumentation]
 * @return 0 in case of malformed parameters
   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~