Fix String::remove warning

Replace strncpy() usage in String::remove method by memmove().
Destination and source shall not overlap when using strncpy() while
memmove() is safer alternative when overlapping.

Warning raised after moved from arm-none-eabi-gcc 6-2017-q2-update
to 8-2018-q4-major:
cores\arduino\WString.cpp: In member function 'void String::remove(unsigned int, unsigned int)':
cores\arduino\WString.cpp:832:10: warning: 'char* strncpy(char*, const char*, size_t)' accessing 0 or more bytes at offsets [0, 2147483647] and [0, 2147483647] may overlap up to 4294967295 bytes at offset [6442450941, 2147483647] [-Wrestrict]
   strncpy(writeTo, buffer + index + count, len - index);
   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/WString.cpp

@@ -829,7 +829,7 @@ void String::remove(unsigned int index, unsigned int count)
   }
   char *writeTo = buffer + index;
   len = len - count;
-  strncpy(writeTo, buffer + index + count, len - index);
+  memmove(writeTo, buffer + index + count, len - index);
   buffer[len] = 0;
 }