Node-splitting models become disconnected

[gertvv]

The following network becomes disconnected when splitting the 1-2 node (thanks to Becca Harvey for reporting):

   treatment study  mean std.dev sampleSize
1          1     1  0.20    5.00         88
2          2     1 -1.10    5.00        172
3          1     2 -0.70    4.47         96
4          2     2 -2.30    4.46        174
5          1     3 -0.30    3.70         63
6          2     3 -1.00    5.90         99
7          1     4 -0.40    4.46         86
8          2     4 -1.70    4.45         85
9          1     5 -0.60    5.60         99
10         2     5 -0.80    7.25         98
11         1     6 -1.10    4.66         57
12         2     6 -0.50    4.59        115
13         1     7 -1.90    4.46        105
14         2     7 -2.80    4.46        104
15         1     8 -1.42    4.64        215
16         2     8 -2.50    4.47        210
17         1     9 -0.60    3.00         36
18         2     9 -2.40    2.96         35
19         1    10  4.50    4.74         15
20         2    10 -1.00    4.70         15
21         1    11 -0.22    3.90        192
22         3    11 -1.32    3.90        201
23         2    12  0.98    3.77         43
24         3    12 -0.43    3.66         47
25         1    13 -0.70    4.40        218
26         2    13 -2.00    4.40        218
27         4    13 -2.30    6.00        222