This fixes a regression from commit f93a2acc:

==20076== Conditional jump or move depends on uninitialised value(s)
==20076==    at 0x402CFF8: bcmp (mc_replace_strmem.c:692)
==20076==    by 0x429DF0B: _cairo_gstate_set_matrix (cairo-gstate.c:722)
==20076==    by 0x428FC52: cairo_set_matrix (cairo.c:1250)
==20076==    by 0x405A11C: _set_rsvg_affine (rsvg-cairo-draw.c:367)
==20076==    by 0x405B941: rsvg_cairo_render_image (rsvg-cairo-draw.c:577)
==20076==    by 0x4058C76: rsvg_render_image (rsvg-base.c:1964)
==20076==    by 0x4039FE8: rsvg_node_image_draw (rsvg-image.c:310)
==20076==    by 0x404FC13: rsvg_node_draw (rsvg-structure.c:69)
==20076==    by 0x4050126: _rsvg_node_draw_children (rsvg-structure.c:87)
==20076==  Uninitialised value was created by a stack allocation
==20076==    at 0x405A236: _set_source_rsvg_pattern (rsvg-cairo-draw.c:190)