runtime: fix string optimization

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6354048

diff --git a/src/pkg/runtime/string.goc b/src/pkg/runtime/string.goc
index 8a5d59b81dcb641a712749db7f6b88f10c5406c9..7cab6d241707941352a76456d2e61ae3739a2c80 100644 (file)
--- a/src/pkg/runtime/string.goc
+++ b/src/pkg/runtime/string.goc
@@ -155,7 +155,9 @@ concatstring(int32 n, String *s)
                        out = s[i];
                }
        }
-       if(count <= 1) // zero or one non-empty string in concatenation
+       if(count == 0)
+               return runtime·emptystring;
+       if(count == 1) // zero or one non-empty string in concatenation
                return out;
        
        out = gostringsize(l);