gas: buffer_and_nest() needs to pass nul-terminated string to temp_ilp()

In 7545aa2dd2 ("gas: improve interaction between read_a_source_file()
and s_linefile()") I didn't pay attention to the dual purpose of the
nul character previously used. This was to a fair degree because of the
open-coding of certain operations. Insert the earlier found line
terminator instead of a hard-coded newline, and do so early in this
special case (bypassing the later general insertion point). Plus
properly use sb_terminate() to mark the end of the string. (Note that
saved_eol_char was misnamed: Without calling sb_terminate() there's
simply random data at that position in the buffer.)

gas/macro.c

@@ -263,13 +263,13 @@ buffer_and_nest (const char *from, const char *to, sb *ptr,
 	  if (from != NULL && strcasecmp (from, "MACRO") == 0
 	      && len >= 8 && strncasecmp (ptr->ptr + i, "linefile", 8) == 0)
 	    {
-	      char saved_eol_char = ptr->ptr[ptr->len];
-
-	      ptr->ptr[ptr->len] = '\n';
-	      temp_ilp (ptr->ptr + i + 8);
+	      sb_add_char (ptr, more);
+	      temp_ilp (sb_terminate (ptr) + i + 8);
 	      s_linefile (0);
 	      restore_ilp ();
-	      ptr->ptr[ptr->len] = saved_eol_char;
+	      line_start = ptr->len;
+	      more = get_line (ptr);
+	      continue;
 	    }
 	}