Don't reference past the end of the vector

An earlier change made find_source_lines read:

    end = &data[size];

However, since 'size' is the size of the vector, this seems fishy.
More obviously ok is to compute the end of the data directly:

    end = data.data () + size;

2018-02-09  Tom Tromey  <tom@tromey.com>

	* source.c (find_source_lines): Don't reference past the end of
	the vector.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2018-02-09  Tom Tromey  <tom@tromey.com>
+
+	* source.c (find_source_lines): Don't reference past the end of
+	the vector.
+
 2018-02-09  Markus Metzger  <markus.t.metzger@intel.com>
 
 	* remote.c (remote_btrace_maybe_reopen): Change error message.

gdb/source.c

@@ -1219,7 +1219,7 @@ find_source_lines (struct symtab *s, int desc)
     size = myread (desc, data.data (), size);
     if (size < 0)
       perror_with_name (symtab_to_filename_for_display (s));
-    end = &data[size];
+    end = data.data () + size;
     p = &data[0];
     line_charpos[0] = 0;
     nlines = 1;