Make compiling a no-op if it's attempted before adding any patterns.

Fixes #485.

Change-Id: Id7900ab2d034432016d425aeceb60a814225f10e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62872
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

re2/filtered_re2.cc

diff --git a/re2/filtered_re2.cc b/re2/filtered_re2.cc
index 49cf686..1ce26a3 100644
--- a/re2/filtered_re2.cc
+++ b/re2/filtered_re2.cc
@@ -70,8 +70,9 @@
     return;
   }
 
+  // Similarly to PrefilterTree::Compile(), make compiling
+  // a no-op if it's attempted before adding any patterns.
   if (re2_vec_.empty()) {
-    LOG(ERROR) << "Compile called before Add.";
     return;
   }
 

re2/prefilter_tree.cc

diff --git a/re2/prefilter_tree.cc b/re2/prefilter_tree.cc
index 3afb241..299a680 100644
--- a/re2/prefilter_tree.cc
+++ b/re2/prefilter_tree.cc
@@ -57,8 +57,9 @@
 
   // Some legacy users of PrefilterTree call Compile() before
   // adding any regexps and expect Compile() to have no effect.
-  if (prefilter_vec_.empty())
+  if (prefilter_vec_.empty()) {
     return;
+  }
 
   compiled_ = true;
 
@@ -272,8 +273,9 @@
     // Some legacy users of PrefilterTree call Compile() before
     // adding any regexps and expect Compile() to have no effect.
     // This kludge is a counterpart to that kludge.
-    if (prefilter_vec_.empty())
+    if (prefilter_vec_.empty()) {
       return;
+    }
 
     LOG(ERROR) << "RegexpsGivenStrings called before Compile.";
     for (size_t i = 0; i < prefilter_vec_.size(); i++)