refactor(IO): clean up ffAppendFDBuffer

for 2 reasons:

- Simplify the Code.
- ffAppendFDBuffer can't make assumptions about how the read data will
  be processed, so the trim calls are inappropriate.

src/util/FFstrbuf.c

@@ -54,6 +54,7 @@ void ffStrbufEnsureFree(FFstrbuf* strbuf, uint32_t free)
     strbuf->allocated = allocate;
 }
 
+// for an empty buffer, free + 1 length memory will be allocated(+1 for the NUL)
 void ffStrbufEnsureFixedLengthFree(FFstrbuf* strbuf, uint32_t free)
 {
     uint32_t oldFree = ffStrbufGetFree(strbuf);
@@ -64,7 +65,7 @@ void ffStrbufEnsureFixedLengthFree(FFstrbuf* strbuf, uint32_t free)
 
     if(strbuf->allocated == 0)
     {
-        newCap += strbuf->length + 1; // +1 for the NUL
+        newCap += strbuf->length + 1;
         char* newbuf = malloc(sizeof(*strbuf->chars) * newCap);
         if(strbuf->length == 0)
             *newbuf = '\0';