dbghelp: Be stricter when looping in symbol tables.

dlls/dbghelp/msc.c

@@ -1322,6 +1322,7 @@ static int codeview_snarf(const struct msc_debug_info* msc_dbg, const BYTE* root
         const union codeview_symbol* sym = (const union codeview_symbol*)(root + i);
         length = sym->generic.len + 2;
         if (i + length > size) break;
+        if (!sym->generic.id || length < 4) break;
         if (length & 3) FIXME("unpadded len %u\n", length);
 
         switch (sym->generic.id)