s_symbol: Close memory leak of symbol name.

Easily done now that s_expression is allocated with talloc. Simply
switch from new to talloc_strdup and the job is done.

This closes the great majority (11263) of the remaining leaks in the
glsl-orangebook-ch06-bump.frag test:

	total heap usage: 55,623 allocs, 55,546 frees
	(was 44,283 frees)

This test is now 99.86% leak-free.
diff --git a/s_expression.cpp b/s_expression.cpp
index 875d739..26be23e 100644
--- a/s_expression.cpp
+++ b/s_expression.cpp
@@ -30,8 +30,8 @@
 
 s_symbol::s_symbol(const char *tmp)
 {
-   this->str = new char [strlen(tmp) + 1];
-   strcpy(this->str, tmp);
+   this->str = talloc_strdup (this, tmp);
+   assert(this->str != NULL);
 }
 
 s_list::s_list()