wingrass: don't try to launch R if not available

          (merge https://trac.osgeo.org/grass/changeset/67175 from trunk)


git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@67176 15284696-431f-4ddb-bdfa-cd5b030d7da7

mswindows/env.bat

@@ -19,5 +19,4 @@ IF EXIST "%ProgramFiles%\RStudio\bin\rstudio.exe" set PATH=%PATH%;%ProgramFiles%
 
 REM set R temporarily to %PATH%
 
-cd %GISBASE%\extrabin
-R path
+IF EXIST "%GISBASE%\extrabin\R.exe" %GISBASE%\extrabin\R path