some undefined behavior issues

[asarubbo]

On 0.5.3, compiled with -fsanitize=undefined:

# amrenc $FILE out.amr                                                                                                                                      
===================================================================                                                                                                                                               
 3GPP AMR Floating-point Speech Encoder  REL-7 V7.0.0 2007-06                                                                                                                                                     
===================================================================                                                                                                                                               
sp_enc.c:3472:12: runtime error: index -28 out of bounds for type 'Float32 [40]'                                                                                                                                  
SUMMARY: AddressSanitizer: undefined-behavior sp_enc.c:3472:12 in                                                                                                                                                 
sp_enc.c:7225:29: runtime error: left shift of negative value -2                                                                                                                                                  
SUMMARY: AddressSanitizer: undefined-behavior sp_enc.c:7225:29 in                                                                                                                                                 
sp_enc.c:7232:27: runtime error: left shift of negative value -273815                                                                                                                                             
SUMMARY: AddressSanitizer: undefined-behavior sp_enc.c:7232:27 in                                                                                                                                                 
sp_enc.c:7149:21: runtime error: left shift of negative value -9208320
SUMMARY: AddressSanitizer: undefined-behavior sp_enc.c:7149:21 in 
[100%]

The issues are reproducible with each wav file.