Fix minor bug in operations.

Note that this bug isn't exposed by the v0.3 API, since it
doesn't allow multi-step signing/verification operations.

Change-Id: I18554e7e1017ed83d3708c134f72cf1d34857437
diff --git a/rsa_operation.cpp b/rsa_operation.cpp
index 1530c70..816c234 100644
--- a/rsa_operation.cpp
+++ b/rsa_operation.cpp
@@ -42,7 +42,7 @@
 }
 
 keymaster_error_t RsaOperation::StoreData(const Buffer& input) {
-    if (!data_.reserve(input.available_read()) ||
+    if (!data_.reserve(data_.available_read() + input.available_read()) ||
         !data_.write(input.peek_read(), input.available_read()))
         return KM_ERROR_MEMORY_ALLOCATION_FAILED;
     return KM_ERROR_OK;