wbmp: getmbi: use unsigned int to avoid undefined shift behavior

oss-fuzz pointed out:
wbmp.c:48:14: runtime error: left shift of 253751679 by 7 places cannot be represented in type 'int'

See previous commit for more details.

src/wbmp.c

@@ -38,7 +38,8 @@
  */
 int getmbi(int (*getin) (void *in), void *in)
 {
-	int i, mbi = 0;
+	unsigned int mbi = 0;
+	int i;
 
 	do {
 		i = getin(in);