When compiling with clang this compilation error is raised:

Making all in rtsp-server
make[3]: Entering directory '/opt/gst/master/gst-rtsp-server/gst/rtsp-server'
  CC       libgstrtspserver_1.0_la-rtsp-onvif-media.lo
rtsp-onvif-media.c:103:16: error: variable 'res' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    } else if (!sinkpad && !gst_caps_is_fixed (caps)) {
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rtsp-onvif-media.c:111:17: note: uninitialized use occurs here
    for (j = 0; res && j < n_caps; j++) {
                ^~~
rtsp-onvif-media.c:103:12: note: remove the 'if' if its condition is always true
    } else if (!sinkpad && !gst_caps_is_fixed (caps)) {
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rtsp-onvif-media.c:103:16: error: variable 'res' is used uninitialized whenever '&&' condition is false [-Werror,-Wsometimes-uninitialized]
    } else if (!sinkpad && !gst_caps_is_fixed (caps)) {
               ^~~~~~~~
rtsp-onvif-media.c:111:17: note: uninitialized use occurs here
    for (j = 0; res && j < n_caps; j++) {
                ^~~
rtsp-onvif-media.c:103:16: note: remove the '&&' if its condition is always true
    } else if (!sinkpad && !gst_caps_is_fixed (caps)) {
               ^~~~~~~~~~~
rtsp-onvif-media.c:82:17: note: initialize the variable 'res' to silence this warning
    gboolean res;
                ^
                 = 0
2 errors generated.