EXECIO handles unset var in stem incorrectly

[RossPatterson]

If the EXECIO command is passed a stem variable with a "gap" (e.g., z.0 = 3; z.1 = 1; z.3=3), it incorrectly uses the preceding variable's value. In VM/SP5, it uses the unset variable's name instead (i.e., Z.2).

Test case:

/* */
trace I
z.0 = 3
z.1 = '1 hi mom'
z.3 = '3 hi dad'
'EXECIO' z.0 'DISKW Z Z A (FINIS STEM Z.'
'TYPE Z Z A'

VM/CE output:

z
     3 *-* z.0 = 3
       >C>   "Z.0"
       >L>   "3"
     4 *-* z.1 = '1 hi mom'
       >C>   "Z.1"
       >L>   "1 hi mom"
     5 *-* z.3 = '3 hi dad'
       >C>   "Z.3"
       >L>   "3 hi dad"
     6 *-* 'EXECIO' z.0 'DISKW Z Z A (FINIS STEM Z.'
       >L>   "EXECIO"
       >C>   "Z.0"
       >V>   "3"
       >O>   "EXECIO 3"
       >L>   "DISKW Z Z A (FINIS STEM Z."
       >O>   "EXECIO 3 DISKW Z Z A (FINIS STEM Z."
     7 *-* 'TYPE Z Z A'
       >L>   "TYPE Z Z A"

1 hi mom
1 hi mom
3 hi dad

     8 *-*
Ready;

VM/SP5 output:

z
     3 *-* z.0 = 3
       >L>   "3"
     4 *-* z.1 = '1 hi mom'
       >L>   "1 hi mom"
     5 *-* z.3 = '3 hi dad'
       >L>   "3 hi dad"
     6 *-* 'EXECIO' z.0 'DISKW Z Z A (FINIS STEM Z.'
       >L>   "EXECIO"
       >V>   "3"
       >O>   "EXECIO 3"
       >L>   "DISKW Z Z A (FINIS STEM Z."
       >O>   "EXECIO 3 DISKW Z Z A (FINIS STEM Z."
     7 *-* 'TYPE Z Z A'
       >L>   "TYPE Z Z A"

1 hi mom
Z.2
3 hi dad

Ready;