Fix exception being thrown on empty glBegin()-glEnd() blocks. Closes #3693.

src/library_gl.js

@@ -5525,6 +5525,7 @@ var LibraryGL = {
 
       // Generate index data in a format suitable for GLES 2.0/WebGL
       var numVertexes = 4 * GLImmediate.vertexCounter / GLImmediate.stride;
+      if (!numVertexes) return;
 #if ASSERTIONS
       assert(numVertexes % 1 == 0, "`numVertexes` must be an integer.");
 #endif

tests/sdlglshader.c

@@ -146,6 +146,10 @@ int main(int argc, char *argv[])
         glTexCoord2f(1, 1  ); glVertex3f(630, 400, 0);
     glEnd();
 
+    // Test https://github.com/kripken/emscripten/issues/3693
+    glBegin( GL_TRIANGLES );
+    glEnd();
+
     SDL_GL_SwapBuffers();
     
 #ifndef __EMSCRIPTEN__