Don't store peeked characters in a char variable

peekNextDigit() returns an int, so it can return -1 in addition to all
256 possible bytes. By putting the result in a signe char, all bytes
over 128 will be interpreted as "no bytes available". Furthermore, it
seems that on SAM "char" is unsigned by default, causing the
"if (c < 0)" line a bit further down to always be false.

Using an int is more appropriate.

A different fix for this issue was suggested in #1399. This fix helps
towards #1728.

cores/arduino/Stream.cpp

@@ -176,7 +176,7 @@ float Stream::parseFloat(char skipChar){
   boolean isNegative = false;
   boolean isFraction = false;
   long value = 0;
-  char c;
+  int c;
   float fraction = 1.0;
 
   c = peekNextDigit();