Invalid ref name normalization leaked memory

When normalizing a reference name, if there is an error because
the name is invalid, then the memory allocated for storing the
name could be leaked if the caller was not careful and assumed
that the error return code meant that no allocation had occurred.

This fixes that by explicitly deallocating the reference name
buffer if there is an error in normalizing the name.

src/refs.c

@@ -1729,6 +1729,9 @@ cleanup:
 			GITERR_REFERENCE,
 			"The given reference name '%s' is not valid", name);
 
+	if (error && normalize)
+		git_buf_free(buf);
+
 	return error;
 }