glsl: fix bounds check in blob_overwrite_bytes

glsl: fix bounds check in blob_overwrite_bytes

clang gives a warning in blob_overwrite_bytes because offset type is
size_t which is unsigned:

src/compiler/glsl/blob.c:110:15: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
   if (offset < 0 || blob->size - offset < to_write)
       ~~~~~~ ^ ~

Remove the less than 0 check to fix this.

Additionally, if offset is greater than blob->size, the 2nd check would
be false due to unsigned math. Rewrite the check to avoid subtraction.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Rob Herring <robh@kernel.org>