Move assertion into if-block.
Move assertion into if-block.

Since the loop always goes up to MAX_VERTEX_ATTRIBS, the ASSERTION may be
triggered even though no more attributes are enabled.

BUG=angle:740

Change-Id: I2bb30d7e9f426ae1a0b8250e65c1fd02107973ac
Reviewed-on: https://chromium-review.googlesource.com/217027
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

csa7mdm/angle,mikolalysenko/angle,android-ia/platform_external_chromium_org_third_party_angle,domokit/waterfall,MIPS/external-chromium_org-third_party-angle,csa7mdm/angle,nandhanurrevanth/angle,larsbergstrom/angle,MIPS/external-chromium_org-third_party-angle,bsergean/angle,ppy/angle,larsbergstrom/angle,android-ia/platform_external_chromium_org_third_party_angle,android-ia/platform_external_chromium_org_third_party_angle,vvuk/angle,ghostoy/angle,nandhanurrevanth/angle,nandhanurrevanth/angle,csa7mdm/angle,MSOpenTech/angle,bsergean/angle,ppy/angle,android-ia/platform_external_chromium_org_third_party_angle,ecoal95/angle,vvuk/angle,mybios/angle,csa7mdm/angle,mlfarrell/angle,ecoal95/angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,mrobinson/rust-angle,crezefire/angle,MIPS/external-chromium_org-third_party-angle,mlfarrell/angle,bsergean/angle,jgcaaprom/android_external_chromium_org_third_party_angle,ghostoy/angle,crezefire/angle,ecoal95/angle,MSOpenTech/angle,mikolalysenko/angle,jgcaaprom/android_external_chromium_org_third_party_angle,jgcaaprom/android_external_chromium_org_third_party_angle,mybios/angle,ecoal95/angle,vvuk/angle,crezefire/angle,domokit/waterfall,ghostoy/angle,nandhanurrevanth/angle,mybios/angle,bsergean/angle,mrobinson/rust-angle,larsbergstrom/angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,jgcaaprom/android_external_chromium_org_third_party_angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,mrobinson/rust-angle,ppy/angle,mrobinson/rust-angle,mybios/angle,larsbergstrom/angle,crezefire/angle,ghostoy/angle,mlfarrell/angle,vvuk/angle,MSOpenTech/angle,ppy/angle,mikolalysenko/angle,geekboxzone/lollipop_external_chromium_org_third_party_angle,mikolalysenko/angle,MSOpenTech/angle,mlfarrell/angle,mrobinson/rust-angle,MIPS/external-chromium_org-third_party-angle,ecoal95/angle