Fix unsafe access to PATH (#756)

* Fix unsafe access to PATH

* Fix leading separator if PATH is empty

auto_cpufreq/core.py

@@ -22,7 +22,10 @@ from auto_cpufreq.power_helper import *
 filterwarnings("ignore")
 
 # add path to auto-cpufreq executables for GUI
-os.environ["PATH"] += ":/usr/local/bin"
+if "PATH" in os.environ:
+    os.environ["PATH"] += os.pathsep + "/usr/local/bin"
+else:
+    os.environ["PATH"] = "/usr/local/bin"
 
 # ToDo:
 # - replace get system/CPU load from: psutil.getloadavg() | available in 5.6.2)
@@ -908,4 +911,4 @@ def not_running_daemon_check():
         exit(1)
     elif IS_INSTALLED_WITH_SNAP and dcheck == "disabled":
         daemon_not_running_msg()
-        exit(1)
+        exit(1)