mesa: fix glRead/DrawBuffer(GL_AUXn) error value

If GL_AUX[123] are passed to glRead/DrawBuffer() when those buffers don't
exist, need to generate GL_INVALID_OPERATION, not GL_INVALID_ENUM.

This regression came from commit 555f0a8818

src/mesa/main/buffers.c

@@ -119,6 +119,10 @@ draw_buffer_enum_to_bitmask(GLenum buffer)
          return BUFFER_BIT_FRONT_LEFT;
       case GL_AUX0:
          return BUFFER_BIT_AUX0;
+      case GL_AUX1:
+      case GL_AUX2:
+      case GL_AUX3:
+         return 1 << BUFFER_COUNT; /* invalid, but not BAD_MASK */
       case GL_COLOR_ATTACHMENT0_EXT:
          return BUFFER_BIT_COLOR0;
       case GL_COLOR_ATTACHMENT1_EXT:
@@ -170,6 +174,10 @@ read_buffer_enum_to_index(GLenum buffer)
          return BUFFER_FRONT_LEFT;
       case GL_AUX0:
          return BUFFER_AUX0;
+      case GL_AUX1:
+      case GL_AUX2:
+      case GL_AUX3:
+         return BUFFER_COUNT; /* invalid, but not -1 */
       case GL_COLOR_ATTACHMENT0_EXT:
          return BUFFER_COLOR0;
       case GL_COLOR_ATTACHMENT1_EXT: