Fixes nvgluCreateFramebuffer().

The framebuffer and renderbuffer did restore in `nvgluCreateFramebuffer()` on succeed. However, they are not restored if error occurs. This commit fixes it.
Good catch!
If the malloc fails (unlikely, though), this will try to restore bad values. Let's change the start of the function to look like this:
```C
 	GLint defaultFBO = 0;
 	GLint defaultRBO = 0;
 	NVGLUframebuffer* fb = NULL;

 	glGetIntegerv(GL_FRAMEBUFFER_BINDING, &defaultFBO);
 	glGetIntegerv(GL_RENDERBUFFER_BINDING, &defaultRBO);

 	fb = (NVGLUframebuffer*)malloc(sizeof(NVGLUframebuffer));
 	if (fb == NULL) goto error;
 	memset(fb, 0, sizeof(NVGLUframebuffer)); 
 	...
```