Fixed nil variable reference in os.findlib() on Linux

samples/project/premake4.lua

@@ -40,4 +40,7 @@ solution "PremakeTestbox"
 		execute     = function ()
 			os.copyfile("premake4.lua", "../premake4.lua")
 		end
-	} 
+	} 
+
+print(os.findlib("Xlib"))
+

src/base/os.lua

@@ -22,7 +22,7 @@
 				path = os.getenv("DYLD_LIBRARY_PATH")
 			else
 				formats = { "lib%s.so", "%s.so" }
-				path = os.getenv("LD_LIBRARY_PATH")
+				path = os.getenv("LD_LIBRARY_PATH") or ""
 		
 				local f = io.open("/etc/ld.so.conf", "r")
 				if f then