Fix out of bounds array access in lexer

If the type char is signed, then in the function
yylex_GetFloatMaskAndFloatLen(), *s can have a negative value and be converted
to a negative int32_t which is then used as an array index. It should be
converted to uint8_t instead to ensure that the value is in the bounds of the
tFloatingFirstChar, tFloatingSecondChar, and tFloatingChars arrays.

test/asm/garbage_char.asm

@@ -0,0 +1 @@
+x�

test/asm/garbage_char.out

@@ -0,0 +1,3 @@
+ERROR: garbage_char.asm(1):
+    syntax error
+error: Assembly aborted (1 errors)!

test/asm/garbage_char.out.pipe

@@ -0,0 +1,3 @@
+ERROR: -(1):
+    syntax error
+error: Assembly aborted (1 errors)!