Don't crash trying to print bytecode for unreachable switch targets

In some circumstances, the label associated with a branch
doesn't get set, and the corresponding branch target ends up pointing
to one byte before the start of the unit's bytecode.

In that case, we were crashing when trying to dump the bytecode.

hphp/runtime/vm/hhbc.cpp

@@ -793,25 +793,29 @@ std::string instrToString(const Opcode* it, const Unit* u /* = NULL */) {
   }                                                               \
 } while (false)
 
-#define READSVEC() do {                     \
-  int sz = readData<int>(it);               \
-  out << " <";                              \
-  const char* sep = "";                     \
-  for (int i = 0; i < sz; ++i) {            \
-    out << sep;                             \
-    if (op == OpSSwitch) {                  \
-      READLITSTR("");                       \
-      out << ":";                           \
-    }                                       \
-    Offset o = readData<Offset>(it);        \
-    if (u != nullptr) {                        \
-      out << u->offsetOf(iStart + o);       \
-    } else {                                \
-      out << o;                             \
-    }                                       \
-    sep = " ";                              \
-  }                                         \
-  out << ">";                               \
+#define READSVEC() do {                         \
+  int sz = readData<int>(it);                   \
+  out << " <";                                  \
+  const char* sep = "";                         \
+  for (int i = 0; i < sz; ++i) {                \
+    out << sep;                                 \
+    if (op == OpSSwitch) {                      \
+      READLITSTR("");                           \
+      out << ":";                               \
+    }                                           \
+    Offset o = readData<Offset>(it);            \
+    if (u != nullptr) {                         \
+      if (iStart + o == u->entry() - 1) {       \
+        out << "Invalid";                       \
+      } else {                                  \
+        out << u->offsetOf(iStart + o);         \
+      }                                         \
+    } else {                                    \
+      out << o;                                 \
+    }                                           \
+    sep = " ";                                  \
+  }                                             \
+  out << ">";                                   \
 } while (false)
 
 #define ONE(a) H_##a