tool_getparam: handle failure of curlx_convert_tchar_to_UTF8()

tool_getparam: handle failure of curlx_convert_tchar_to_UTF8()

Reported by GCC analyzer:

Error: GCC_ANALYZER_WARNING (CWE-476):
src/tool_getparam.c: scope_hint: In function 'parse_args'
src/tool_getparam.c:2318:38: warning[-Wanalyzer-possible-null-dereference]: dereference of possibly-NULL 'orig_opt'
lib/curlx.h:56: included_from: Included from here.
src/tool_getparam.c:28: included_from: Included from here.
lib/curl_multibyte.h:70:51: note: in definition of macro 'curlx_convert_tchar_to_UTF8'
src/tool_getparam.c:2316:16: note: in expansion of macro 'curlx_convert_tchar_to_UTF8'

Reviewed-by: Marcel Raad
Reviewed-by: Daniel Stenberg
Closes #7023