Use more std::string in fastboot.

Change-Id: Ic8c77eac1e0088627ab650050d9c97c5749e5c39
diff --git a/fastboot/engine.cpp b/fastboot/engine.cpp
index 47567c0..db5d0e0 100644
--- a/fastboot/engine.cpp
+++ b/fastboot/engine.cpp
@@ -351,21 +351,21 @@
         }
         if (a->op == OP_DOWNLOAD) {
             status = fb_download_data(transport, a->data, a->size);
-            status = a->func(a, status, status ? fb_get_error() : "");
+            status = a->func(a, status, status ? fb_get_error().c_str() : "");
             if (status) break;
         } else if (a->op == OP_COMMAND) {
             status = fb_command(transport, a->cmd);
-            status = a->func(a, status, status ? fb_get_error() : "");
+            status = a->func(a, status, status ? fb_get_error().c_str() : "");
             if (status) break;
         } else if (a->op == OP_QUERY) {
             status = fb_command_response(transport, a->cmd, resp);
-            status = a->func(a, status, status ? fb_get_error() : resp);
+            status = a->func(a, status, status ? fb_get_error().c_str() : resp);
             if (status) break;
         } else if (a->op == OP_NOTICE) {
             fprintf(stderr,"%s\n",(char*)a->data);
         } else if (a->op == OP_DOWNLOAD_SPARSE) {
             status = fb_download_data_sparse(transport, reinterpret_cast<sparse_file*>(a->data));
-            status = a->func(a, status, status ? fb_get_error() : "");
+            status = a->func(a, status, status ? fb_get_error().c_str() : "");
             if (status) break;
         } else if (a->op == OP_WAIT_FOR_DISCONNECT) {
             transport->WaitForDisconnect();