Fix handling of extern symbols in phi_function

In 646cf29941, initialisation of dynamic objects was added, but extern
symbols were not considered. Includes a test that fails without this fix.

regression/cbmc/extern3/main.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+
+extern int x;
+
+int main(int argc, char *argv[])
+{
+  if(argc > 5)
+    x = 42;
+
+  __CPROVER_assert(x == 42, "should fail");
+}

regression/cbmc/extern3/test.desc

@@ -0,0 +1,10 @@
+CORE
+main.c
+
+^EXIT=10$
+^SIGNAL=0$
+^VERIFICATION FAILED$
+--
+--
+The change to phi_function of 646cf29941499 failed to consider the case of
+extern variables, which we leave uninitialised.

src/goto-symex/symex_goto.cpp

@@ -473,11 +473,11 @@ static void merge_names(
     rhs = goto_state_rhs;
   else if(goto_state.guard.is_false())
     rhs = dest_state_rhs;
-  else if(goto_count == 0)
+  else if(goto_count == 0 && symbol.value.is_not_nil())
   {
     rhs = dest_state_rhs;
   }
-  else if(dest_count == 0)
+  else if(dest_count == 0 && symbol.value.is_not_nil())
   {
     rhs = goto_state_rhs;
   }