LibGfx: Make TextTagData verify input is 7-bit ASCII

TextTagData::from_bytes() errors out if that isn't the case, but
let's make sure other potential future callers get it right too.

Userland/Libraries/LibGfx/ICC/TagTypes.h

@@ -762,6 +762,8 @@ public:
         : TagData(offset, size, Type)
         , m_text(move(text))
     {
+        for (u8 byte : text.bytes())
+            VERIFY(byte < 128);
     }
 
     // Guaranteed to be 7-bit ASCII.