[FYI] Fix crash in machoread.c

[Tom Tromey <tom@tromey.com>]

"./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.
---
 gdb/ChangeLog   | 4 ++++
 gdb/machoread.c | 6 ++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index be45bc31fa2..a125b72e420 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-06-27  Tom Tromey  <tom@tromey.com>
+
+	* machoread.c (macho_symfile_read): Define "symbol_table" earlier.
+
 2018-06-27  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* gdb-gdb.py.in: Format using autopep8.
diff --git a/gdb/machoread.c b/gdb/machoread.c
index 4c1d4f0bfa7..07e1cdaf4f8 100644
--- a/gdb/machoread.c
+++ b/gdb/machoread.c
@@ -793,6 +793,9 @@ macho_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
   bfd *abfd = objfile->obfd;
   long storage_needed;
   std::vector<oso_el> oso_vector;
+  /* We have to hold on to the symbol table until the call to
+     macho_symfile_read_all_oso at the end of this function.  */
+  gdb::def_vector<asymbol *> symbol_table;
 
   /* Get symbols from the symbol table only if the file is an executable.
      The symbol table of object files is not relocated and is expected to
@@ -812,8 +815,7 @@ macho_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
 	{
 	  long symcount;
 
-	  gdb::def_vector<asymbol *> symbol_table (storage_needed
-						   / sizeof (asymbol *));
+	  symbol_table.resize (storage_needed / sizeof (asymbol *));
 
           minimal_symbol_reader reader (objfile);
 
-- 
2.17.1