XkbVirtualModsToReal should only fail to set mask if the server does not support XKB, but it still made Oracle Parfait complain:
Error: Uninitialised memory
Uninitialised memory variable [uninitialised-mem-var] (CWE 457):
Possible access to uninitialised memory referenced by variable 'mask'
at line 863 of lib/libX11/src/xkb/XKBMisc.c in function 'XkbUpdateKeyTypeVirtualMods'.
Path in callee avoiding write at line 862
mask allocated at line 860