mesa: fix accidental regression in GLSL built-in texture matrix lookup

Was broken by commit 9aca9a4b72b2a7b378e50bd88f9c3324d07375ec.

(cherry picked from commit fe984aed5a)

Conflicts:

	src/mesa/shader/slang/slang_builtin.c

src/mesa/shader/slang/slang_builtin.c

@@ -107,10 +107,14 @@ lookup_statevar(const char *var, GLint index1, GLint index2, const char *field,
    if (isMatrix) {
       if (tokens[0] == STATE_TEXTURE_MATRIX) {
          if (index1 >= 0) {
-            tokens[1] = index1;
+            tokens[1] = index1; /* which texture matrix */
             index1 = 0; /* prevent extra addition at end of function */
          }
       }
+      if (index1 < 0) {
+         /* index1 is unused: prevent extra addition at end of function */
+         index1 = 0;
+      }
    }
    else if (strcmp(var, "gl_DepthRange") == 0) {
       tokens[0] = STATE_DEPTH_RANGE;