Return error from boot_find_key instead of asserting

When the hash key length parameter is invalid (bigger than allowed) just
return an error for the caller routines; this will work like a not found
key and allows the removal of assert() code in this module.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/image_validate.c b/boot/bootutil/src/image_validate.c
index a496bc4..b883e44 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -21,7 +21,6 @@
  * Modifications are Copyright (c) 2019 Arm Limited.
  */
 
-#include <assert.h>
 #include <stddef.h>
 #include <inttypes.h>
 #include <string.h>
@@ -183,7 +182,9 @@
     const struct bootutil_key *key;
     uint8_t hash[32];
 
-    assert(keyhash_len <= 32);
+    if (keyhash_len > 32) {
+        return -1;
+    }
 
     for (i = 0; i < bootutil_key_cnt; i++) {
         key = &bootutil_keys[i];