mesa: Prevent buffer underrun when handling MESA_GL_EXTENSION_OVERRIDE.

diff --git a/src/mesa/main/extensions.c b/src/mesa/main/extensions.c
index 5f2c74a758e7681624f2988ba2c94a8a35a1e9db..a843a40925c95fb584f50ab1b8e4144759eda616 100644 (file)
--- a/src/mesa/main/extensions.c
+++ b/src/mesa/main/extensions.c
@@ -754,7 +754,7 @@ get_extension_override( struct gl_context *ctx )
 
    /* Remove trailing space. */
    len = strlen(extra_exts);
-   if (extra_exts[len - 1] == ' ')
+   if (len > 0 && extra_exts[len - 1] == ' ')
       extra_exts[len - 1] = '\0';
 
    return extra_exts;