Use of uninitialized values

[ODCA117]

Make pjsua_verify_sip_url() initialzied url to 0.

When running Valgrind in our apps we got the following issues reported:

==3012942== 3 errors in context 1 of 12:
==3012942== Conditional jump or move depends on uninitialised value(s)
==3012942==    at 0x49E18B3: pj_ansi_strxcpy (in ../git/libsipd_check.so)
==3012942==    by 0x49293AA: pjsua_verify_sip_url (in ../git/libsipd_check.so)

and was able to track it down to url not being initialized in pjsua_verify_sip_url() but still being used in pj_ansi_strxcpy().
Fixed it by using calloc instead of alloc.