[gdb/testsuite] Fix gdb.base/jit-bfd-name.exp

When running test-case gdb.base/jit-bfd-name.exp, I run into:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.base/jit-bfd-name.exp.
ERROR: can't read "start": no such variable
...

The problem is that commit c96ceed ("gdb: include the end address in
in-memory bfd filenames") introduced a use of variable start, but not a
definition.

Fix this by adding the missing definition.

Tested on x86_64-linux.

Author: vries

gdb/testsuite/gdb.base/jit-bfd-name.exp

@@ -135,7 +135,7 @@ foreach addr $symfile_addrs len $symfile_lengths {
 set count 1
 foreach addr $symfile_addrs len $symfile_lengths {
     # Drop any loading zeros from the symfile address.
-    set addr [format 0x%x $addr]
+    set start [format 0x%x $addr]
 
     # Calculate the end address.
     set end [format 0x%x [expr $addr + $len]]