glsl: fix crash on glsl macro redefinition

In case shader contains two equal macro defines, first one with trailing spaces
and the second one without.
`#define A 1   `
`#define A 1`
The parser crashes

Fixes: 0346ad3774 ("glsl: ignore trailing whitespace when define redefined")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5312>

src/compiler/glsl/glcpp/glcpp-parse.y

@@ -1187,6 +1187,9 @@ _token_list_equal_ignoring_space(token_list_t *a, token_list_t *b)
             node_b = node_b->next;
       }
 
+      if (node_a == NULL && node_b == NULL)
+         break;
+
       if (node_b == NULL && node_a->token->type == SPACE) {
          while (node_a && node_a->token->type == SPACE)
             node_a = node_a->next;