Check for null result from gdb_demangle

I am sending this patch on behalf of kmoy@google.com, who discovered the bug
and wrote the fix.

gdb_demangle can return null for strings that don't properly demangle. The null
check was mistakenly removed in commit 43816ebc33. Without this check, GDB
aborts when loading symbols from some binaries.

gdb/ChangeLog
2020-02-21  Ali Tamur  <tamur@google.com>

	* dwarf2/read.c (dwarf2_name): Add null check.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-02-21  Ali Tamur  <tamur@google.com>
+
+	* dwarf2/read.c (dwarf2_name): Add null check.
+
 2020-02-20  Tom Tromey  <tom@tromey.com>
 
 	* dwarf2/read.c (dwarf2_find_containing_comp_unit): Use ">", not

gdb/dwarf2/read.c

@@ -21756,6 +21756,8 @@ dwarf2_name (struct die_info *die, struct dwarf2_cu *cu)
 	    {
 	      gdb::unique_xmalloc_ptr<char> demangled
 		(gdb_demangle (DW_STRING (attr), DMGL_TYPES));
+	      if (demangled == nullptr)
+		return nullptr;
 
 	      const char *base;