[processing] recover changes introduced in b928d16, lost on refactoring and merging

Author: volaya

python/plugins/processing/algs/grass7/Grass7Utils.py

@@ -254,7 +254,7 @@ def prepareGrass7Execution(commands):
             Grass7Utils.createGrass7BatchJobFileFromGrass7Commands(commands)
             os.chmod(Grass7Utils.grassBatchJobFilename(), stat.S_IEXEC
                      | stat.S_IREAD | stat.S_IWRITE)
-            if isMac():
+            if isMac() and os.path.exists(Grass7Utils.grassPath() + os.sep + 'grass70.sh'):
                 command = Grass7Utils.grassPath() + os.sep + 'grass70.sh ' \
                     + Grass7Utils.grassMapsetFolder() + '/PERMANENT'
             else: