Add some g_return_if_fail()s to soup-headers.c
Add some g_return_if_fail()s to soup-headers.c

(partly from "cache" branch)

jwendell/libsoup,Distrotech/libsoup,Distrotech/libsoup,Distrotech/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,fanc999/libsoup,fanc999/libsoup,jwendell/libsoup,fanc999/libsoup,fanc999/libsoup,Distrotech/libsoup,ahmedammar/platform_external_gst_libsoup,ahmedammar/platform_external_gst_libsoup,Distrotech/libsoup,jwendell/libsoup,ahmedammar/platform_external_gst_libsoup