Fix uninitialized return value in MTP module

The rc variable in the get_file function in the libmtp-based
module was not initialized, leading to a random return code when
no MTP device is present. This could lead to a crash if the
returned value happened to be zero, which is interpreted as a
success by the calling code (which then assumes that the share
has been successfully retrieved).

src/scheme-libmtp.c

@@ -277,7 +277,7 @@ gfsec_scheme_libmtp_get_file(gfsec_scheme_t   scheme,
 {
     LIBMTP_mtpdevice_t *device;
     struct getfile_cb_data cb_data;
-    int rc;
+    int rc = GFSEC_SCHEME_STATUS_ERROR;
 
     if ( scheme != GFSEC_SCHEME_MTP )
         return GFSEC_SCHEME_STATUS_UNSUPPORTED_SCHEME;