glx: Fix out-of-bounds reads from negative return

The callers of these functions were casting -1 to unsigned and then
using 4GB indexes. By returning 0 we match all the other size functions.

GLX size functions return -1 to indicate error, but GL size functions return 0.

Signed-off-by: Nathan Kidd <nkidd@rocketsoftware.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1647>

glx/singlesize.c

@@ -105,7 +105,7 @@ __glGetMap_size(GLenum target, GLenum query)
         }
         break;
     }
-    return -1;
+    return 0;
 }
 
 GLint
@@ -164,7 +164,7 @@ __glGetPixelMap_size(GLenum map)
         query = GL_PIXEL_MAP_A_TO_A_SIZE;
         break;
     default:
-        return -1;
+        return 0;
     }
     glGetIntegerv(query, &size);
     return size;