Don't crash if dwarf_decode_macro_bytes's 'body' is NULL

Hi,

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1708192
      https://bugzilla.redhat.com/show_bug.cgi?id=1708786

During the Fedora RPM build process, gdb-add-index is invoked to
extract the DWARF index from the binary, and GDB will segfault because
dwarf2read.c:parse_definition_macro's 'body' variable is NULL.

The underlying problem is that Fedora's rpm-build's "debugedit"
program will silently corrupt .debug_macro strings when a binary is
compiled with -g3.  This is being taken care of by Mark Wielaard,
here:

  https://bugzilla.redhat.com/show_bug.cgi?id=1708786

However, I still feel it's important to make GDB more resilient
against invalid DWARF input, so I'm proposing this rather simple patch
to catch the situation when "body == NULL" (i.e., it's probably been
corrupted) and issue a complaint.  This is not a real fix to the
problem, of course, but at least GDB is able to finish without
segfaulting.

OK for master?

gdb/ChangeLog:
2019-05-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1708192
	* dwarf2read.c (dwarf_decode_macro_bytes): Check whether 'body' is
	NULL, and complain if that's the case.

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -24627,7 +24627,21 @@ dwarf_decode_macro_bytes (struct dwarf2_cu *cu,
 			 is_define ? _("definition") : _("undefinition"),
 			 line == 0 ? _("zero") : _("non-zero"), line, body);
 
-	    if (is_define)
+	    if (body == NULL)
+	      {
+		/* Fedora's rpm-build's "debugedit" binary
+		   corrupted .debug_macro sections.
+
+		   For more info, see
+		   https://bugzilla.redhat.com/show_bug.cgi?id=1708786 */
+		complaint (_("debug info gives %s invalid macro %s "
+			     "without body (corrupted?) at line %d "
+			     "on file %s"),
+			   at_commandline ? _("command-line") : _("in-file"),
+			   is_define ? _("definition") : _("undefinition"),
+			   line, current_file->filename);
+	      }
+	    else if (is_define)
 	      parse_macro_definition (current_file, line, body);
 	    else
 	      {

