terminal: Cope with unreleased versions of tmux

When tmux is built from a non-release Git version, its version number is "next-X" where X appears to be the expected version number for the next release. For example, when built from the current state of master, running "tmux -V" yields:

tmux next-2.9

Currently check_tmux_pane_size only checks for the version being less than 1.9, so it seems unfair to fail with an obscure Python error in this case.

Let's just use the version number after the "next-" prefix if it is present.

---
 meta/lib/oe/terminal.py | 2 ++
 1 file changed, 2 insertions(+)

index 94afe394ed..bf5c37f897 100644
@@ -296,6 +296,8 @@ def check_terminal_version(terminalName):
             vernum = ver.split(' ')[-1]
         if ver.startswith('tmux'):
             vernum = ver.split()[-1]
+        if ver.startswith('tmux next-'):
+            vernum = ver.split()[-1][5:]
     return vernum
 
 def distro_name():

2.19.1