libbacktrace: don't free ELF strtab if error occurs after saving syminfo

Fixes #29

elf.c

@@ -3011,6 +3011,7 @@ elf_add (struct backtrace_state *state, const char *filename, int descriptor,
 	 string table permanently.  */
       backtrace_release_view (state, &symtab_view, error_callback, data);
       symtab_view_valid = 0;
+      strtab_view_valid = 0;
 
       *found_sym = 1;