Check negative return of PyList_Size
    
    Merges:
    https://pagure.io/libuser/issue/28
    
    In case of an error, PyList_Size can return a negative value. We should
    check that case, also to avoid compiler warnings like:
    
    Error: COMPILER_WARNING: [#def41] [warning: defect not occurring in libuser-0.60-9.el7]
    libuser-0.62/python/misc.c: scope_hint: In function 'libuser_admin_prompt'
    libuser-0.62/python/misc.c:160:12: warning: argument 1 range [9223372036854775808, 18446744073709551615] exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
    /usr/include/glib-2.0/glib/glist.h:32: included_from: Included from here.
    /usr/include/glib-2.0/glib/ghash.h:33: included_from: Included from here.
    /usr/include/glib-2.0/glib.h:50: included_from: Included from here.
    libuser-0.62/python/misc.c:25: included_from: Included from here.
    /usr/include/glib-2.0/glib/gmem.h:96:10: note: in a call to allocation function 'g_malloc0_n' declared here