Fix bugs in bam_aux_update_str()

s/m_data/data/ and remove casts that were suppressing a warning that
would have diagnosed this mistake.  Resample bam_get_aux(), which may
also be changed by the realloc().

Author: jenniferliddle

sam.c

@@ -1304,17 +1304,16 @@ int bam_aux_update_str(bam1_t *b, const char tag[2], int len, const char *data)
     bam_aux_del(b,s);
     s -= 2;
     int l_aux = bam_get_l_aux(b);
-    uint8_t *aux = bam_get_aux(b);
 
     b->l_data += 3 + len;
     if (b->m_data < b->l_data) {
-        ptrdiff_t s_offset = (ptrdiff_t) (s - b->m_data);
+        ptrdiff_t s_offset = s - b->data;
         b->m_data = b->l_data;
         kroundup32(b->m_data);
         b->data = (uint8_t*)realloc(b->data, b->m_data);
-        s = (uint8_t *) (b->m_data + s_offset);
+        s = b->data + s_offset;
     }
-    memmove(s+3+len, s, l_aux - (s - aux));
+    memmove(s+3+len, s, l_aux - (s - bam_get_aux(b)));
     s[0] = tag[0];
     s[1] = tag[1];
     s[2] = type;