Fixed bug in parent revision [832a1994c7] unpredictably breaking the
execution of precompiled bytecode (i.e. tbcload'ed bytecode).

The semi-inversion of a guard condition dropped the part about
TCL_BYTECODE_PRECOMPILED of the original condition, allowing the core
to attempt to recompile code for which there are no script sources
(anymore), which then fails.
Merged fix to bytecode engine problem. Details in merged revision.