Fix rrd_cgi.c Cppcheck_warnings

Fix rrd_cgi.c Cppcheck_warnings

Remove the identical inner 'if' condition.
Use the %zu format specifier for size_t returned from strlen(),
according to: https://en.cppreference.com/w/c/string/byte/strlen

- Fixes the following Cppcheck warnings:
  [src/rrd_cgi.c:349] (warning) Identical inner 'if' condition is
    always true (outer condition is 'calcpr[i]' and inner condition is
    'calcpr[i]'). [identicalInnerCondition]

  [src/rrd_cgi.c:527] (portability) %zd in format string (no. 1)
    requires 'ssize_t' but the argument type is 'size_t
    {aka unsigned long}'. [invalidPrintfArgType_sint]
  or
  [src/rrd_cgi.c:527] (portability) %zd in format string (no. 1)
    requires 'ssize_t' but the argument type is 'size_t
    {aka unsigned long long}'. [invalidPrintfArgType_sint]