Fix invalid reads by memcmp.

Str may be smaller than Start->Name here. Use strncmp to avoid scanning past the
end. Found by valgrind.

llvm-svn: 165157
diff --git a/clang/lib/Driver/OptTable.cpp b/clang/lib/Driver/OptTable.cpp
index 680ea99..257f353 100644
--- a/clang/lib/Driver/OptTable.cpp
+++ b/clang/lib/Driver/OptTable.cpp
@@ -162,7 +162,7 @@
   for (; Start != End; ++Start) {
     // Scan for first option which is a proper prefix.
     for (; Start != End; ++Start)
-      if (memcmp(Str, Start->Name, strlen(Start->Name)) == 0)
+      if (strncmp(Str, Start->Name, strlen(Start->Name)) == 0)
         break;
     if (Start == End)
       break;