Fix crash in machoread.c

"./gdb ./gdb" was crashing for me on macOS.  Investigating showed that
macho_symfile_read was crashing because "symbol_table" was being freed
too soon.  This was introduced by my earlier patch to change
macho_symfile_read to use a std::vector.

Tested on macOS 10.13.5 using "./gdb ./gdb".  This should un-break
various already existing tests (testsuite/gdb.gdb at least), so no new
test case.

I'm checking this in as obvious.

gdb/ChangeLog
2018-06-27  Tom Tromey  <tom@tromey.com>

	* machoread.c (macho_symfile_read): Define "symbol_table" earlier.