Make celt_pitch_xcorr_edsp() work on ARMv5TE.

We were assuming that LDR, LDRD, and STRD could be used on
 unaligned addresses, but this turns out not to be true on really
 old hardware.
diff --git a/celt/pitch.c b/celt/pitch.c
index 2d63a5a..d2b3054 100644
--- a/celt/pitch.c
+++ b/celt/pitch.c
@@ -253,6 +253,11 @@
 celt_pitch_xcorr_c(const opus_val16 *_x, const opus_val16 *_y, opus_val32 *xcorr, int len, int max_pitch)
 {
    int i,j;
+   /*The EDSP version requires that max_pitch is at least 1, and that _x is
+      32-bit aligned.
+     Since it's hard to put asserts in assembly, put them here.*/
+   celt_assert(max_pitch>0);
+   celt_assert((((unsigned char *)_x-(unsigned char *)NULL)&3)==0);
 #ifdef FIXED_POINT
    opus_val32 maxcorr=1;
 #endif