Fix warnings: ex_cmds2.c: do_source(): Np dereference: FP.

Problem    : Dereference of null pointer @ 2462.
Diagnostic : False positive.
Rationale  : Error occurred if neither loop neither following if were
             entered (this implied `script_items.ga_len < 0`, which
             should not be possible).
Resolution : Assert not negative length (loop or if entered).
Fix warnings: ex_cmds2.c: do_source(): Unitialized arg (2): MI.

Problem    : Uninitialized argument value @ 2485.
             Uninitialized argument value @ 2507.
Diagnostic : Multithreading issues.
Rationale  : Error can only occur if globals `do_profiling`, `time_fd`
             are modified while function is executing.
Resolution : Use local copy of globals.