x86_64: int32_to_float_fmul_scalar sign extend integer length

libavcodec/x86/fmtconvert.asm

@@ -37,6 +37,9 @@ cglobal int32_to_float_fmul_scalar, 4, 4, %1, dst, src, mul, len
     SWAP 0, 2
 %elif ARCH_X86_32
     movss   m0, mulm
+%endif
+%if ARCH_X86_64
+    movsxd  lenq, lend
 %endif
     SPLATD  m0
     shl     lenq, 2