Avoid calling memchr with a zero range as this is undefined behavior.

src/string-search.h

Index: src/string-search.h
diff --git a/src/string-search.h b/src/string-search.h
index 09bc36ef82ea0e6f5e436da10d529d20248b4d4f..c8b2f155087485e65d33eec4b2f4de71352a7fc9 100644
--- a/src/string-search.h
+++ b/src/string-search.h
@@ -102,6 +102,17 @@ class StringSearch : private StringSearchBase {
     return -1;
   }
 
+  static inline const SubjectChar* SafeMemChr(const SubjectChar* string,
+                                              PatternChar pattern_char,
+                                              size_t search_length) {
+    if (search_length == 0) {
+      return NULL;
+    } else {
+      return reinterpret_cast<const SubjectChar*>(
+          memchr(string, pattern_char, search_length));
+    }
+  }
+
   static int SingleCharSearch(StringSearch<PatternChar, SubjectChar>* search,
                               Vector<const SubjectChar> subject,
                               int start_index);
@@ -200,10 +211,8 @@ int StringSearch<PatternChar, SubjectChar>::SingleCharSearch(
   PatternChar pattern_first_char = search->pattern_[0];
   int i = index;
   if (sizeof(SubjectChar) == 1 && sizeof(PatternChar) == 1) {
-    const SubjectChar* pos = reinterpret_cast<const SubjectChar*>(
-        memchr(subject.start() + i,
-               pattern_first_char,
-               subject.length() - i));
+    const SubjectChar* pos = SafeMemChr(subject.start() + i, pattern_first_char,
+                                        subject.length() - i);
     if (pos == NULL) return -1;
     return static_cast<int>(pos - subject.start());
   } else {
@@ -256,10 +265,8 @@ int StringSearch<PatternChar, SubjectChar>::LinearSearch(
   int n = subject.length() - pattern_length;
   while (i <= n) {
     if (sizeof(SubjectChar) == 1 && sizeof(PatternChar) == 1) {
-      const SubjectChar* pos = reinterpret_cast<const SubjectChar*>(
-          memchr(subject.start() + i,
-                 pattern_first_char,
-                 n - i + 1));
+      const SubjectChar* pos =
+          SafeMemChr(subject.start() + i, pattern_first_char, n - i + 1);
       if (pos == NULL) return -1;
       i = static_cast<int>(pos - subject.start()) + 1;
     } else {
@@ -507,10 +514,8 @@ int StringSearch<PatternChar, SubjectChar>::InitialSearch(
     badness++;
     if (badness <= 0) {
       if (sizeof(SubjectChar) == 1 && sizeof(PatternChar) == 1) {
-        const SubjectChar* pos = reinterpret_cast<const SubjectChar*>(
-            memchr(subject.start() + i,
-                   pattern_first_char,
-                   n - i + 1));
+        const SubjectChar* pos =
+            SafeMemChr(subject.start() + i, pattern_first_char, n - i + 1);
         if (pos == NULL) {
           return -1;
         }