CLEANUP: ebtree: remove a few annoying signedness warnings

(from ebtree 6.0.6)

Care has been taken not to make the code bigger (it even got smaller
due to a possible simplification).
(cherry picked from commit 7a2c1df646049c7daac52677ec11ed63048cd150)
diff --git a/ebtree/ebmbtree.h b/ebtree/ebmbtree.h
index 1df3fcf..336b6c1 100644
--- a/ebtree/ebmbtree.h
+++ b/ebtree/ebmbtree.h
@@ -527,7 +527,7 @@
 			 * bits, let's compare prefixes and descend the cover
 			 * subtree if they match.
 			 */
-			if (node->node.bit >> 1 == pfx)
+			if ((unsigned short)node->node.bit >> 1 == pfx)
 				troot = node->node.branches.b[EB_LEFT];
 			else
 				troot = node->node.branches.b[EB_RGHT];
diff --git a/ebtree/ebtree.h b/ebtree/ebtree.h
index e0f9a18..96138f1 100644
--- a/ebtree/ebtree.h
+++ b/ebtree/ebtree.h
@@ -376,7 +376,7 @@
 	eb_troot_t    *node_p;  /* link node's parent */
 	eb_troot_t    *leaf_p;  /* leaf node's parent */
 	short int      bit;     /* link's bit position. */
-	short int      pfx;     /* data prefix length, always related to leaf */
+	short unsigned int pfx; /* data prefix length, always related to leaf */
 };
 
 /* Return the structure of type <type> whose member <member> points to <ptr> */