main: create_buffers unlocks mutex when throwing OUT_OF_MEMORY.

Ilia Mirkin found that I had forgotten to free the mutex in the error case.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/main/bufferobj.c

@@ -1333,6 +1333,7 @@ create_buffers(GLsizei n, GLuint *buffers, bool dsa)
          buf = ctx->Driver.NewBufferObject(ctx, buffers[i]);
          if (!buf) {
             _mesa_error(ctx, GL_OUT_OF_MEMORY, "%s", func);
+            mtx_unlock(&ctx->Shared->Mutex);
             return;
          }
       }