Initialize a variable to suppress compiler error

GCC (mips32 Buildbot) is complaining about the possibility of
uninitialized usage.

Change-Id: I217aa371189e06bed498288bf40e85406806c5ff
diff --git a/runtime/monitor.cc b/runtime/monitor.cc
index 2c30828..ed6a8e4 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -221,7 +221,7 @@
                                           size_t num_waiters) {
   DCHECK(owner != nullptr);
   const char* owners_filename;
-  int32_t owners_line_number;
+  int32_t owners_line_number = 0;
   std::string name;
   owner->GetThreadName(name);
   if (owners_method != nullptr) {