dlmalloc: ensure gd is set for early alloc

Attempting to run the sandbox leads to a segfault, because some dynamic
libraries (outside of u-boot) attempt to use malloc() to allocate memory
before u-boot's gd variable is initialized.

Check for gd not being NULL in the SYS_MALLOC_F_LEN handling, so that
malloc() doesn't crash when called at this point.

 $ gdb -q --args ./u-boot
 (gdb) r
 Program received signal SIGSEGV, Segmentation fault.
 0x0000000000412b9b in malloc (bytes=bytes@entry=37) at common/dlmalloc.c:2184
 2184		if (!(gd->flags & GD_FLG_RELOC)) {
 (gdb) p gd
 $1 = (gd_t *) 0x0
 (gdb) bt
 #0  0x0000000000412b9b in malloc (bytes=bytes@entry=37) at common/dlmalloc.c:2184
 #1  0x00007ffff75bf8e1 in set_binding_values (domainname=0x7ffff11f4f12 "libgpg-error", dirnamep=0x7fffffffe168, codesetp=0x0)
     at bindtextdom.c:228
 #2  0x00007ffff75bfb4c in set_binding_values (codesetp=0x0, dirnamep=0x7fffffffe168, domainname=<optimized out>) at bindtextdom.c:350
 #3  __bindtextdomain (domainname=<optimized out>, dirname=0x7ffff11f4f00 "/usr/share/locale") at bindtextdom.c:348
 #4  0x00007ffff11eca17 in ?? () from /lib/x86_64-linux-gnu/libgpg-error.so.0
 #5  0x00007ffff7dea9fa in call_init (l=<optimized out>, argc=argc@entry=1, argv=argv@entry=0x7fffffffe208,
     env=env@entry=0x7fffffffe218) at dl-init.c:78
 #6  0x00007ffff7deaae3 in call_init (env=0x7fffffffe218, argv=0x7fffffffe208, argc=1, l=<optimized out>) at dl-init.c:36
 #7  _dl_init (main_map=0x7ffff7ffe1a8, argc=1, argv=0x7fffffffe208, env=0x7fffffffe218) at dl-init.c:126
 #8  0x00007ffff7ddd1ca in _dl_start_user () from /lib64/ld-linux-x86-64.so.2

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Simon Glass <sjg@chromium.org>

common/dlmalloc.c

@@ -2181,7 +2181,7 @@ Void_t* mALLOc(bytes) size_t bytes;
   INTERNAL_SIZE_T nb;
 
 #ifdef CONFIG_SYS_MALLOC_F_LEN
-	if (!(gd->flags & GD_FLG_RELOC)) {
+	if (gd && !(gd->flags & GD_FLG_RELOC)) {
 		ulong new_ptr;
 		void *ptr;