Boot: Fix GNUARM compiler warning

Fix a false positive compiler warning to not
break the MINSIZEREL build.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: I1dc5f5c5dceda7034803027c114bcf4cd1ac8590
diff --git a/bl2/ext/mcuboot/bootutil/src/loader.c b/bl2/ext/mcuboot/bootutil/src/loader.c
index 6aecfce..f91dc8f 100644
--- a/bl2/ext/mcuboot/bootutil/src/loader.c
+++ b/bl2/ext/mcuboot/bootutil/src/loader.c
@@ -2666,7 +2666,7 @@
     int fa_id;
     uint32_t boot_sequence[BOOT_NUM_SLOTS];
     uint32_t img_cnt;
-    struct image_header *selected_image_header;
+    struct image_header *selected_image_header = NULL;
 #ifdef MCUBOOT_RAM_LOADING
     int image_copied = 0;
     uint32_t img_dst = 0;