tools/msximage.c: fix warning about nptr possibly uninitialized

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/tools/mxsimage.c b/tools/mxsimage.c
index 0d5b0ea..04beefe 100644
--- a/tools/mxsimage.c
+++ b/tools/mxsimage.c
@@ -1416,7 +1416,7 @@
 {
 	char *tok;
 	char *line = cmd->cmd;
-	char *rptr;
+	char *rptr = NULL;
 	int ret;
 
 	/* Analyze the identifier on this line first. */