test/librados/c_read_operations.cc: fix invalid checks

Wrong variable for return of rados_getxattrs_next() and
rados_omap_get_next() used (len vs. val_len).

Fix for:

CID 1219464 (#1 of 1): Logically dead code (DEADCODE)
 dead_error_line: Execution cannot reach this expression
 key == NULL inside statement if (len == 0UL && key == NU...

CID 1219465 (#1 of 1): Logically dead code (DEADCODE)
 dead_error_line: Execution cannot reach this expression
 key == NULL inside statement if (len == 0UL && key == NU...

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>