fix selected warnings from cppcheck

Fixes issue 2.

R=r
CC=re2-dev
http://codereview.appspot.com/665041
diff --git a/re2/compile.cc b/re2/compile.cc
index 5bab033..8f6ef36 100644
--- a/re2/compile.cc
+++ b/re2/compile.cc
@@ -205,8 +205,7 @@
 }
 
 Compiler::~Compiler() {
-  if (prog_)
-    delete prog_;
+  delete prog_;
 }
 
 bool Compiler::CanAllocInst() {