texgetimage: add missing return on error

Missed this back in the arb_robustness branch
<6b329b9274b18c50f4177eef7ee087d50ebc1525>.

NOTE: This is a candidate for the 7.11 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/texgetimage.c

@@ -851,6 +851,7 @@ getcompressedteximage_error_check(struct gl_context *ctx, GLenum target,
          _mesa_error(ctx, GL_INVALID_OPERATION,
                      "glGetnCompressedTexImageARB(out of bounds access:"
                      " bufSize (%d) is too small)", clientMemSize);
+         return GL_TRUE;
       }
    } else {
       /* do bounds checking on PBO write */