test: init err to 0 in sdp test (cppcheck)

[alfredh]

$ cppcheck --version
Cppcheck 2.12.0

$ cppcheck -q -f --std=c11 .

src/main/main.c:704:6: error: Uninitialized variable: n [uninitvar]
 if (n < 0)
     ^
src/main/main.c:789:11: error: Uninitialized variable: fd [uninitvar]
  index = fd;
          ^
src/sipevent/sipevent.h:51:39: error: syntax error [syntaxError]
void sipnot_refresh(struct sipnot *not, uint32_t expires);
                                      ^
test/sdp.c:367:9: warning: Uninitialized variable: err [uninitvar]
 return err;
        ^
test/sdp.c:330:13: note: Assuming condition is false
 for (i=0; i<RE_ARRAY_SIZE(msgs); i++) {
            ^
test/sdp.c:367:9: note: Uninitialized variable: err
 return err;
        ^