backtrace: trim of unneeded slash from solib

Previously we would output solib with an unnecessary slash:

  native: art::JDWP::JdwpState::Run()+6018 [0x416bf3e3] (/libart.so)

Trim it off so it omits the slash:

  native: art::JDWP::JdwpState::Run()+6018 [0x41961f6b] (libart.so)

Change-Id: I82baf1e624227858f5750ef57058474d21777451
diff --git a/runtime/utils.cc b/runtime/utils.cc
index d8f8f8f..237d217 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -1049,7 +1049,7 @@
   if (last_slash == std::string::npos) {
     return map->name;
   }
-  return map->name.substr(last_slash);
+  return map->name.substr(last_slash + 1);
 }
 
 void DumpNativeStack(std::ostream& os, pid_t tid, const char* prefix, bool include_count) {