Examples: imgui_impl_opengl3: Fix empty printout on shader load. (#2584)

Fixed minor bug in CheckShader and CheckProgram

The log_length reported by 
glGetProgramiv(handle, GL_INFO_LOG_LENGTH, &log_length)
will at least return 1, since the string delimiter is also counted.

The old version would always print and empty string to stderr. This is annoying in the emscripten port, since it prints a red error message to the Javascript console. The new version fixes this behavior.

examples/imgui_impl_opengl3.cpp

@@ -384,7 +384,7 @@ static bool CheckShader(GLuint handle, const char* desc)
     glGetShaderiv(handle, GL_INFO_LOG_LENGTH, &log_length);
     if ((GLboolean)status == GL_FALSE)
         fprintf(stderr, "ERROR: ImGui_ImplOpenGL3_CreateDeviceObjects: failed to compile %s!\n", desc);
-    if (log_length > 0)
+    if (log_length > 1)
     {
         ImVector<char> buf;
         buf.resize((int)(log_length + 1));
@@ -402,7 +402,7 @@ static bool CheckProgram(GLuint handle, const char* desc)
     glGetProgramiv(handle, GL_INFO_LOG_LENGTH, &log_length);
     if ((GLboolean)status == GL_FALSE)
         fprintf(stderr, "ERROR: ImGui_ImplOpenGL3_CreateDeviceObjects: failed to link %s! (with GLSL '%s')\n", desc, g_GlslVersionString);
-    if (log_length > 0)
+    if (log_length > 1)
     {
         ImVector<char> buf;
         buf.resize((int)(log_length + 1));