Fix for PR70492

[Marcel Böhme]

Hi,

This fixes the invalid write of size 8 detailed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70492

Handle the special case when consume_count returns -1 due to an integer overflow when parsing the length of the virtual table qualifier in cplus-dem.c:2994 (gnu_special).

Index: libiberty/cplus-dem.c
===================================================================
--- libiberty/cplus-dem.c	(revision 234663)
+++ libiberty/cplus-dem.c	(working copy)
@@ -3001,6 +3001,11 @@ gnu_special (work, mangled, declp)
 		      success = 1;
 		      break;
 		    }
+                  else if (n == -1)
+                    {
+                      success = 0;
+                      break;
+                    }
 		}
 	      else
 		{