Fix invalid reads by memcmp.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165157 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/OptTable.cpp

@@ -162,7 +162,7 @@ Arg *OptTable::ParseOneArg(const ArgList &Args, unsigned &Index) const {
   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;