Fix some default_cc behavior

Fixes Homebrew/homebrew#10245.
Fixes Homebrew/homebrew#10248.

Library/Homebrew/utils.rb

@@ -269,7 +269,7 @@ module MacOS extend self
   def default_cc
     cc = `/usr/bin/xcrun -find cc 2> /dev/null`.chomp
     cc = "#{dev_tools_path}/cc" if cc.empty?
-    Pathname.new(cc).realpath.basename.to_s
+    Pathname.new(cc).realpath.basename.to_s rescue nil
   end
 
   def default_compiler
@@ -304,10 +304,9 @@ module MacOS extend self
     end
   end
 
-  # usually /Developer
   def xcode_prefix
     @xcode_prefix ||= begin
-      path = `/usr/bin/xcode-select -print-path 2>&1`.chomp
+      path = `/usr/bin/xcode-select -print-path 2>/dev/null`.chomp
       path = Pathname.new path
       if path.directory? and path.absolute?
         path